US20220328089A1 - Semiconductor element memory device - Google Patents
Semiconductor element memory device Download PDFInfo
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Definitions
- the present invention relates to a semiconductor memory device including a semiconductor element.
- Typical planar MOS transistors include a channel that extends in a horizontal direction along the upper surface of the semiconductor substrate.
- SGTs include a channel that extends in a direction perpendicular to the upper surface of the semiconductor substrate (see, for example, PTL 1 and NPL 1). Accordingly, the density of semiconductor devices can be made higher with SGTs than with planar MOS transistors.
- Such SGTs can be used as selection transistors to implement highly integrated memories, such as a DRAM (Dynamic Random Access Memory, see, for example, NPL 2) to which a capacitor is connected, a PCM (Phase Change Memory, see, for example, NPL 3) to which a resistance change element is connected, an RRAM (Resistive Random Access Memory, see, for example, NPL 4), and an MRAM (Magneto-resistive Random Access Memory, see, for example, NPL 5) that changes the resistance by changing the orientation of a magnetic spin with a current.
- a DRAM memory cell see, for example, NPL 6) constituted by a single MOS transistor and including no capacitor.
- the present application relates to a dynamic flash memory that can be constituted only by a MOS transistor and that includes no resistance change element or capacitor.
- FIGS. 7A-7D illustrate a write operation of a DRAM memory cell constituted by a single MOS transistor and including no capacitor described above
- FIGS. 8A and 8B illustrate a problem in the operation
- FIGS. 9A-9C illustrate a read operation (see NPL 7 to NPL 10).
- FIG. 7A illustrates a “1” write state.
- the memory cell is formed on an SOI substrate 100 , is constituted by a source N + layer 103 (hereinafter, a semiconductor region that contains a donor impurity in high concentrations is referred to as “N + layer”) to which a source line SL is connected, a drain N + layer 104 to which a bit line BL is connected, a gate conductor layer 105 to which a word line WL is connected, and a floating body 102 of a MOS transistor 110 , and includes no capacitor.
- the single MOS transistor 110 constitutes the DRAM memory cell. Directly under the floating body 102 , a SiO 2 layer 101 of the SOI substrate is in contact with the floating body 102 .
- the MOS transistor 110 is operated in the saturation region. That is, a channel 107 , for electrons, extending from the source N + layer 103 has a pinch-off point 108 and does not reach the drain N + layer 104 to which the bit line is connected.
- a high voltage is applied to both the bit line BL connected to the drain N + layer and the word line WL connected to the gate conductor layer 105 , and the MOS transistor 110 is operated at the gate voltage that is about one-half of the drain voltage, the electric field intensity becomes maximum at the pinch-off point 108 that is in the vicinity of the drain N + layer 104 .
- FIG. 7B illustrates a state in which the floating body 102 is charged to saturation with the generated positive holes 106 .
- FIG. 7C illustrates a state of rewriting from the “1” write state to a “0” write state.
- the voltage of the bit line BL is set to a negative bias, and the PN junction between the drain N + layer 104 and the P layer, namely, the floating body 102 , is forward biased.
- the positive holes 106 in the floating body 102 generated in advance in the previous cycle flow into the drain N + layer 104 that is connected to the bit line BL.
- the two memory cells are in a state in which the memory cell 110 ( FIG. 7B ) is filled with the generated positive holes 106 , and from the memory cell 110 ( FIG. 7C ), the generated positive holes are discharged.
- the potential of the floating body 102 of the memory cell 110 filled with the positive holes 106 becomes higher than that of the floating body 102 in which generated positive holes are not present. Therefore, the threshold voltage for the memory cell 110 to which “1” is written becomes lower than the threshold voltage for the memory cell 110 to which “0” is written. This is illustrated in FIG. 7D .
- the capacitance C FB of the floating body is equal to the sum of the capacitance C WL between the gate to which the word line is connected and the floating body, the junction capacitance C SL of the PN junction between the source N + layer 103 to which the source line is connected and the floating body 102 , and the junction capacitance C BL of the PN junction between the drain N + layer 104 to which the bit line is connected and the floating body 102 and is expressed as follows.
- the capacitive coupling ratio ⁇ WL between the gate to which the word line is connected and the floating body is expressed as follows.
- ⁇ WL C WL /( C WL +C BL +C SL ) (11)
- a change in the word line voltage V WL at the time of reading or writing affects the voltage of the floating body 102 that functions as a storage node (contact point) of the memory cell. This is illustrated in FIG. 8B .
- the voltage VFB of the floating body 102 rises from V FB1 , which is the voltage in the initial state before the word line voltage changes, to V FB2 due to capacitive coupling with the word line.
- the voltage change amount ⁇ V FB is expressed as follows.
- FIGS. 9A -9C illustrate a read operation where FIG. 9( a ) illustrates a “1” write state and FIG. 9B illustrates a “0” write state.
- Vb is set for the floating body 102 to write “1”
- the floating body 102 is lowered to a negative bias.
- the floating body 102 is lowered to a further negative bias, and it is difficult to provide a sufficiently large margin to the potential difference between “1” and “0” at the time of writing as illustrated in FIG. 9C . Therefore, there has been difficulty in commercially introducing DRAM memory cells actually including no capacitor.
- NPL 12 N. Loubet, et al.: “Stacked Nanosheet Gate-All-Around Transistor to Enable Scaling Beyond FinFET”, 2017 IEEE Symposium on VLSI Technology Digest of Technical Papers, T17-5, T230-T231, June 2017.
- capacitor-less single-transistor DRAMs In capacitor-less single-transistor DRAMs (gain cells), capacitive coupling between the word line and the floating body is strong. When the potential of the word line is changed at the time of data reading or at the time of data writing, the change is transmitted as direct noise to the floating body, which is a problem. This causes a problem of erroneous reading or erroneous rewriting of storage data and makes it difficult to commercially introduce capacitor-less single-transistor DRAMs (gain cells).
- a semiconductor element memory device is
- a memory device including a plurality of pages arranged in a column direction, each of the pages being constituted by a plurality of memory cells arranged in a row direction on a substrate,
- each memory cell included in each of the pages including:
- a semiconductor base material that stands on the substrate in a vertical direction or that extends in a horizontal direction along the substrate;
- a first gate insulating layer that partially or entirely surrounds a side surface of the semiconductor base material between the first impurity layer and the second impurity layer and that is in contact with or in close vicinity to the first impurity layer;
- a second gate insulating layer that surrounds the side surface of the semiconductor base material, that is connected to the first gate insulating layer, and that is in contact with or in close vicinity to the second impurity layer;
- first gate conductor layer that partially or entirely covers the first gate insulating layer
- the first page is constituted by at least one or more sets of split pages, a first group of memory cells included in the first page performs a split-page write operation in a first period, and a second group of memory cells included in the first page performs the split-page write operation in a second period (second invention).
- a third group of memory cells included in a second page among the plurality of pages performs the split-page write operation in a third period
- a fourth group of memory cells included in the second page performs the split-page write operation in a fourth period
- the first period is followed by the third period, the second period, and the fourth period in a time-series order
- a semiconductor element memory device includes a block in which the plurality of pages of the memory device according to any of the first to third inventions described above are arranged, in which
- the first impurity layer of the memory cell included in the plurality of pages is connected to a source line
- the second impurity layer thereof is connected to a bit line
- one of the first gate conductor layer or the second gate conductor layer thereof is connected to a word line
- the other of the first gate conductor layer or the second gate conductor layer is connected to a first driving control line
- the source line is connected to the semiconductor base materials in the block, and
- a first gate capacitance between the first gate conductor layer and the channel semiconductor layer is larger than a second gate capacitance between the second gate conductor layer and the channel semiconductor layer (sixth invention).
- the first gate conductor layer is isolated into two conductor layers around the first gate insulating layer in plan view (seventh invention).
- a group of positive holes generated by an impact ionization phenomenon are retained inside the channel semiconductor layer, and a voltage of the channel semiconductor layer is made equal to a first data retention voltage that is higher than the voltage of either the first impurity layer or the second impurity layer or that is higher than the voltages of both the first impurity layer and the second impurity layer, and
- the voltages applied to the first impurity layer, the second impurity layer, the first gate conductor layer, and the second gate conductor layer are controlled to discharge the group of positive holes through either the first impurity layer or the second impurity layer or both the first impurity layer and the second impurity layer, and the voltage of the channel semiconductor layer is made equal to a second data retention voltage that is lower than the first data retention voltage (eighth invention).
- FIG. 1 is a structural diagram of a memory device including an SGT according to a first embodiment.
- FIGS. 2A, 2B and 2C are diagrams for explaining an effect attained in a case where the gate capacitance of a first gate conductor layer 5 a connected to a plate line PL is made larger than the gate capacitance of a second gate conductor layer 5 b to which a word line WL is connected in the memory device including an SGT according to the first embodiment.
- FIG. 3B are diagrams for explaining the mechanism of the write operation of the memory device including an SGT according to the first embodiment.
- FIG. 4A is a diagram for explaining a mechanism of a page erase operation of the memory device including an SGT according to the first embodiment.
- FIGS. 4BA, 4BB, 4BC and 4BD are diagrams for explaining the mechanism of the page erase operation of the memory device including an SGT according to the first embodiment.
- FIG. 4C are diagrams for explaining the mechanism of the page erase operation of the memory device including an SGT according to the first embodiment.
- FIGS. 4DA, 4DB, 4DC and 4DD are diagrams for explaining a mechanism of the page erase operation of the memory device including an SGT according to the first embodiment.
- FIGS. 4EA, 4EB, 4EC and 4ED are diagrams for explaining a mechanism of the page erase operation of the memory device including an SGT according to the first embodiment.
- FIGS. 5A, 5B and 5C are diagrams for explaining a mechanism of a read operation of the memory device including an SGT according to the first embodiment.
- FIG. 6A is a circuit block diagram for explaining a split-page write operation of the memory device including an SGT according to the first embodiment.
- FIG. 6B is an operation waveform diagram for explaining the split-page write operation of the memory device including an SGT according to the first embodiment.
- FIG. 6C is a diagram for explaining the split-page write operation of the memory device including an SGT according to the first embodiment.
- FIG. 6D is a diagram for explaining the split-page write operation of the memory device including an SGT according to the first embodiment.
- FIG. 6E is a diagram for explaining the split-page write operation of the memory device including an SGT according to the first embodiment.
- FIGS. 7A, 7B, 7C and 7D are diagrams for explaining a write operation of a DRAM memory cell including no capacitor in the related art.
- FIGS. 8A and 8B are diagrams for explaining a problem in the operation of the DRAM memory cell including no capacitor in the related art.
- FIGS. 9A, 9B and 9C are diagrams for explaining a read operation of the DRAM memory cell including no capacitor in the related art.
- a dynamic flash memory including a semiconductor element according to the present invention
- FIG. 1 The structure and operation mechanisms of a dynamic flash memory cell according to a first embodiment of the present invention will be described with reference to FIG. 1 to FIGS. 5A-5C .
- the structure of the dynamic flash memory cell will be described with reference to FIG. 1 .
- An effect attained in a case where the gate capacitance of a first gate conductor layer 5 a connected to a plate line PL is made larger than the gate capacitance of a second gate conductor layer 5 b to which a word line WL is connected will be described with reference to FIGS. 2A-2C .
- a mechanism of a data write operation will be described with reference to FIGS. 3AA-3AC and FIG. 3B
- mechanisms of a data erase operation will be described with reference to FIGS. 4A to FIGS. 4EA-4ED
- a mechanism of a data read operation will be described with reference to FIGS. 5A-5C .
- FIG. 1 illustrates the structure of the dynamic flash memory cell according to the first embodiment of the present invention.
- Si pillar which is an example of “semiconductor base material” in the claims) of the P or i (intrinsic) conductivity type formed on a substrate
- N + layers 3 a and 3 b which are examples of “first impurity layer” and “second impurity layer” in the claims, one of which functions as the source and the other functions as the drain, are formed respectively.
- the part of the Si pillar 2 between the N + layers 3 a and 3 b that function as the source and the drain functions as a channel region 7 (which is an example of “channel semiconductor layer” in the claims).
- a first gate insulating layer 4 a (which is an example of “first gate insulating layer” in the claims) and a second gate insulating layer 4 b (which is an example of “second gate insulating layer” in the claims) are formed.
- the first gate insulating layer 4 a and the second gate insulating layer 4 b are in contact with or in close vicinity to the N + layers 3 a and 3 b that function as the source and the drain respectively.
- the first gate conductor layer 5 a (which is an example of “first gate conductor layer” in the claims) and the second gate conductor layer 5 b (which is an example of “second gate conductor layer” in the claims) are formed respectively.
- the first gate conductor layer 5 a and the second gate conductor layer 5 b are isolated from each other by an insulating layer 6 (which is also referred to as “first insulating layer”).
- the channel region 7 between the N + layers 3 a and 3 b is constituted by a first channel Si layer 7 a (which is also referred to as “first channel semiconductor layer”) surrounded by the first gate insulating layer 4 a and a second channel Si layer 7 b (which is also referred to as “second channel semiconductor layer”) surrounded by the second gate insulating layer 4 b.
- first channel semiconductor layer which is also referred to as “first channel semiconductor layer”
- second channel Si layer 7 b which is also referred to as “second channel semiconductor layer” surrounded by the second gate insulating layer 4 b.
- the N + layers 3 a and 3 b that function as the source and the drain, the channel region 7 , the first gate insulating layer 4 a, the second gate insulating layer 4 b, the first gate conductor layer 5 a, and the second gate conductor layer 5 b constitute a dynamic flash memory cell 10 .
- the N + layer 3 a that functions as the source is connected to a source line SL (which is an example of “source line” in the claims), the N + layer 3 b that functions as the drain is connected to a bit line BL (which is an example of “bit line” in the claims), the first gate conductor layer 5 a is connected to the plate line PL (which is an example of “first driving control line” in the claims), and the second gate conductor layer 5 b is connected to the word line WL (which is an example of “word line” in the claims).
- the structure is such that the gate capacitance of the first gate conductor layer 5 a to which the plate line PL is connected is larger than the gate capacitance of the second gate conductor layer 5 b to which the word line WL is connected.
- the gate length of the first gate conductor layer 5 a is made longer than the gate length of the second gate conductor layer 5 b.
- the thicknesses of the respective gate insulating layers may be made different such that the thickness of the gate insulating film of the first gate insulating layer 4 a is thinner than the thickness of the gate insulating film of the second gate insulating layer 4 b.
- the dielectric constants of the materials of the respective gate insulating layers may be made different such that the dielectric constant of the gate insulating film of the first gate insulating layer 4 a is higher than the dielectric constant of the gate insulating film of the second gate insulating layer 4 b.
- the gate capacitance of the first gate conductor layer 5 a connected to the plate line PL may be made larger than the gate capacitance of the second gate conductor layer 5 b to which the word line WL is connected, by a combination of any of the lengths of the gate conductor layers 5 a and 5 b and the thicknesses and dielectric constants of the gate insulating layers 4 a and 4 b.
- FIGS. 2A-2C are diagrams for explaining an effect attained in a case where the gate capacitance of the first gate conductor layer 5 a connected to the plate line PL is made larger than the gate capacitance of the second gate conductor layer 5 b to which the word line WL is connected.
- FIG. 2A is a simplified structural diagram of the dynamic flash memory cell according to the first embodiment of the present invention and illustrates only main parts.
- the bit line BL, the word line WL, the plate line PL, and the source line SL are connected, and the potential state of the channel region 7 is determined by the voltage states of the lines.
- FIG. 2B is a diagram for explaining the capacitance relationships of the respective lines.
- the capacitance CFB of the channel region 7 is equal to the sum of the capacitance C WL between the gate conductor layer 5 b to which the word line WL is connected and the channel region 7 , the capacitance CPL between the gate conductor layer 5 a to which the plate line PL is connected and the channel region 7 , the junction capacitance C SL of the PN junction between the source N + layer 3 a to which the source line SL is connected and the channel region 7 , and the junction capacitance C BL of the PN junction between the drain N + layer 3 b to which the bit line BL is connected and the channel region 7 , and is expressed as follows.
- the coupling ratio ⁇ WL between the word line WL and the channel region 7 the coupling ratio ⁇ PL between the plate line PL and the channel region 7 , the coupling ratio ⁇ BL between the bit line BL and the channel region 7 , and the coupling ratio ⁇ SL between the source line SL and the channel region 7 are expressed as follows.
- ⁇ WL C WL /( C WL +C PL +C BL +C SL ) (2)
- ⁇ PL C PL /( C WL +C PL +C BL +C SL ) (3)
- ⁇ BL C BL /( C WL +C PL +C BL +C SL ) (4)
- ⁇ SL C SL /( C WL +C PL +C BL +C SL ) (5)
- FIG. 2C is a diagram for explaining a change in the voltage V FB of the channel region 7 when the voltage V WL of the word line WL rises at the time of a read operation or a write operation and subsequently drops.
- the potential difference ⁇ V FB when the voltage V WL of the word line WL rises from 0 V to a high voltage state V WLH and the voltage V FB of the channel region 7 transitions from a low voltage state V FBL to a high voltage state VFBH is expressed as follows.
- V FBH - V FBL ⁇ WL ⁇ V WLH ( 6 )
- the coupling ratio ⁇ WL between the word line WL and the channel region 7 is small and the coupling ratio ⁇ PL between the plate line PL and the channel region 7 is large, and therefore, ⁇ V FB is small, and the voltage V FB of the channel region 7 negligibly changes even when the voltage V WL of the word line WL changes at the time of a read operation or a write operation.
- FIGS. 3AA-3AC and FIG. 3B illustrate a memory write operation (which is an example of “page write operation” in the claims) of the dynamic flash memory cell according to the first embodiment of the present invention.
- FIG. 3AA illustrates a mechanism of the write operation
- FIG. 3AB illustrates operation waveforms of the bit line BL, the source line SL, the plate line PL, the word line WL, and the channel region 7 that functions as a floating body FB.
- the dynamic flash memory cell is in a “0” erase state, and the voltage of the channel region 7 is equal to V FB “0”.
- Vss is applied to the bit line BL, the source line SL, and the word line WL, and VBLL is applied to the plate line PL.
- Vss is equal to 0 V and V PLL is equal to 2 V.
- Vss is equal to 0 V
- V PLL is equal to 2 V.
- the word line WL rises from Vss to VvgLli. Accordingly, when the threshold voltage for “0” erase for a second N-channel MOS transistor region that is a region in which the second gate conductor layer 5 b to which the word line WL is connected surrounds the channel region 7 b is denoted by Vt WL “0”, as the voltage of the word line WL rises, in a range from Vss to Vt WL “0”, the voltage of the channel region 7 becomes equal to V FB “0”+ ⁇ BL ⁇ V BLH + ⁇ WL ⁇ Vt WL “0” due to second capacitive coupling between the word line WL and the channel region 7 .
- an inversion layer 12 b in a ring form is formed in the channel region 7 on the inner periphery of the second gate conductor layer 5 b and interrupts the second capacitive coupling between the word line WL and the channel region 7 .
- a first N-channel MOS transistor region formed of the channel region 7 a that is surrounded by the first gate conductor layer 5 a operates in the saturation region.
- the second N-channel MOS transistor region including the second gate conductor layer 5 b to which the word line WL is connected operates in the linear region.
- a pinch-off point is not present in the channel region 7 on the inner periphery of the second gate conductor layer 5 b to which the word line WL is connected, and the inversion layer 12 b is formed on the entire inner periphery of the gate conductor layer 5 b.
- the inversion layer 12 b that is formed on the entire inner periphery of the second gate conductor layer 5 b to which the word line WL is connected substantially functions as the drain of the first N-channel MOS transistor region.
- the electric field becomes maximum in a first boundary region of the channel region 7 between the first N-channel MOS transistor region including the first gate conductor layer 5 a and the second N-channel MOS transistor region including the second gate conductor layer 5 b that are connected in series, and an impact ionization phenomenon occurs in this region.
- This region is a source-side region when viewed from the second N-channel MOS transistor region including the second gate conductor layer 5 b to which the word line WL is connected, and therefore, this phenomenon is called a source-side impact ionization phenomenon.
- This source-side impact ionization phenomenon electrons flow from the N + layer 3 a to which the source line SL is connected toward the N + layer 3 b to which the bit line is connected.
- the accelerated electrons collide with lattice Si atoms, and electron-positive hole pairs are generated by the kinetic energy. Although some of the generated electrons flow into the first gate conductor layer 5 a and into the second gate conductor layer 5 b, most of the generated electrons flow into the N + layer 3 b to which the bit line BL is connected (not illustrated).
- a generated group of positive holes 9 (which is an example of “group of positive holes” in the claims) are majority carriers in the channel region 7 , with which the channel region 7 is charged to a positive bias.
- the N + layer 3 a to which the source line SL is connected is at 0 V, and therefore, the channel region 7 is charged up to the built-in voltage Vb (about 0.7 V) of the PN junction between the N + layer 3 a to which the source line SL is connected and the channel region 7 .
- Vb about 0.7 V
- the capacitive coupling between the word line WL and the channel region 7 is substantially formed only during a period from when the word line WL drops to Vt WL “1” or below to when the word line WL drops to Vss.
- the voltage of the channel region 7 becomes equal to Vb ⁇ WL ⁇ Vt WL “1”.
- Vt WL “1” is lower than Vt WL “0” described above, and ⁇ WL ⁇ Vt WL “1” is small.
- V FB “1 ” Vb ⁇ WL ⁇ Vt WL “1” ⁇ BL ⁇ V BLH (7)
- the coupling ratio ⁇ BL between the bit line BL and the channel region 7 is also small. Accordingly, as illustrated in FIG. 3B , the threshold voltage for the second N-channel MOS transistor region of the second channel region 7 b to which the word line WL is connected decreases.
- electron-positive hole pairs may be generated by an impact ionization phenomenon in a second boundary region between the first impurity layer 3 a and the first channel semiconductor layer 7 a or in a third boundary region between the second impurity layer 3 b and the second channel semiconductor layer 7 b instead of the first boundary region, and the channel region 7 may be charged with the generated group of positive holes 9 .
- a mechanism of a memory erase operation (which is an example of “page erase operation” in the claims) will be described with reference to FIG. 4A to FIGS. 4EA-4ED .
- FIG. 4A is a memory block circuit diagram for explaining a page erase operation. Although nine memory cells CL 11 to CL 33 in three rows and three columns are illustrated, the actual memory block is larger than this matrix. When memory cells are arranged in a matrix, one of the directions of the arrangement is called a row direction (or in rows) and the direction perpendicular to the one of the directions is called “column direction” (or in columns). To each of the memory cells, the source line SL, a corresponding one of the bit lines BL 1 to BL 3 , a corresponding one of the plate lines PL 1 to PL 3 , and a corresponding one of the word lines WL 1 to WL 3 are connected. For example, it is assumed that memory cells CL 21 to CL 23 to which the plate line PL 2 and the word line WL 2 are connected are selected in this block and a page erase operation is performed.
- FIG. 4BA is a timing operation waveform diagram of main nodes in the erase operation.
- T 0 to T 12 indicate times from the start to the end of the erase operation.
- FIG. 4BB illustrates a state at time T 0 before the erase operation, in which the group of positive holes 9 generated by impact ionization in the previous cycle are stored in the channel region 7 .
- bit lines BL 1 to BL 3 and the source line SL rise from Vss to V BLH and V SLH respectively and are in a high-voltage state.
- Vss is, for example, equal to 0 V.
- the plate line PL 2 selected in the page erase operation rises from a first voltage V PLL to a second voltage V PLH and is in a high-voltage state
- the word line WL 2 selected in the page erase operation rises from a third voltage Vss to a fourth voltage V WLH and is in a high-voltage state, and this prevents the inversion layer 12 a on the inner periphery of the first gate conductor layer 5 a to which the plate line PL 2 is connected and the inversion layer 12 b on the inner periphery of the second gate conductor layer 5 b to which the word line WL 2 is connected from being formed in the channel region 7 .
- V tWL and V tPL the threshold voltage for the second N-channel MOS transistor region on the side of the word line WL 2 and the threshold voltage for the first N-channel MOS transistor region on the side of the plate line PL 2
- V tWL and V tPL the threshold voltage for the second N-channel MOS transistor region on the side of the word line WL 2
- V tPL the threshold voltage for the first N-channel MOS transistor region on the side of the plate line PL 2
- V tNL and V tPL are equal to 0.5 V
- V WLH and V PLH need to be set to 3 V
- V BLH and V SLH need to be set to 3.5 V or higher.
- This voltage rise is possible because the voltage of the bit lines BL 1 to BL 3 and that of the source line SL are high voltages of V BLH and V SLH respectively and the PN junction between the source N + layer 3 a and the channel region 7 and the PN junction between the drain N + layer 3 b and the channel region 7 are in a reverse bias state accordingly.
- the voltage V FB of the channel region 7 becomes equal to the built-in voltage Vb of the PN junction formed by the source N + layer 3 a and the P layer, namely, the channel region 7 , and the PN junction formed by the drain N + layer 3 b and the P layer, namely, the channel region 7 .
- the voltage VFB of the channel region 7 efficiently changes from Vb to VFB“0” due to the first capacitive coupling between the plate line PL 2 and the channel region 7 and the second capacitive coupling between the word line WL 2 and the channel region 7 without the inversion layer 12 a on the side of the plate line PL 2 or the inversion layer 12 b on the side of the word line WL 2 being formed in the channel region 7 .
- the voltage difference ⁇ V FB of the channel region 7 between the “1” write state and the “0” erase state is expressed by the following expressions.
- V FB ⁇ “ 1 ” Vb - ⁇ WL ⁇ Vt WL ⁇ “ 1 ” - ⁇ BL ⁇ V BLH ( 7 )
- V FB ⁇ “ 0 ” Vb - ⁇ WL ⁇ V WLH - ⁇ PL ⁇ ( V PLH - V PLL ) ( 8 )
- the sum of ⁇ WL and ⁇ PL is greater than or equal to 0.8, ⁇ V FB is large, and a sufficient margin is provided.
- the threshold voltage on the side of the plate line PL 2 is high due to a substrate bias effect. Therefore, when the voltage applied to the plate line PL 2 is set to, for example, the threshold voltage or lower, the first N-channel MOS transistor region on the side of the plate line PL 2 becomes non-conducting and does not allow the memory cell current to flow therethrough. This state is illustrated in the right part of FIG. 4C and indicated as “PL: non-conducting”.
- bit lines BL 1 to BL 3 and the source line SL slightly decrease the voltage of the channel region 7 due to capacitive coupling, this decrease is equal to the increase in the voltage of the channel region 7 by the bit lines BLi to BL 3 and the source line SL due to capacitive coupling from time T 7 to time T 8 , and therefore, the decrease and the increase in the voltage by the bit lines BL 1 to BL 3 and the source line SL are canceled out, and the voltage of the channel region 7 is not affected consequently.
- a second data retention voltage which is an example of “second data retention voltage” in the claims.
- FIGS. 4DA-4DD are different from FIGS. 4BA-4BD in that the bit lines BL 1 to BL 3 are kept at Vss or put in a floating state and the word line WL 2 is kept at Vss during the page erase operation. Accordingly, from time T 1 to time T 2 , even when the source line SL rises from Vss to V SLH , the second N-channel MOS transistor region of the word line WL 2 is non-conducting, and the memory cell current does not flow therethrough. Therefore, the group of positive holes 9 caused by an impact ionization phenomenon are not generated. The others are the same as in FIG.
- the source line SL changes between Vss and V SLH
- the plate line PL 2 changes between V PLL and V PLH .
- the group of positive holes 9 are discharged to the first impurity layer, namely, the N + layer 3 a, of the source line SL.
- FIGS. 4EA-4ED are different from FIGS. 4BA-4BD in that the source line SL is kept at Vss or put in a floating state and the plate line PL 2 is kept at Vss during the page erase operation. Accordingly, from time T 1 to time T 2 , even when the bit lines BL 1 to BL 3 rise from Vss to VBLH, the first N-channel MOS transistor region of the plate line PL 2 is non-conducting, and the memory cell current does not flow therethrough. Therefore, the group of positive holes 9 caused by an impact ionization phenomenon are not generated. The others are the same as in FIGS.
- the group of positive holes 9 are discharged to the second impurity layer, namely, the N + layer 3 b, of the bit lines BL 1 to BL 3 .
- FIGS. 5A-5C are diagrams for explaining a read operation of the dynamic flash memory cell according to the first embodiment of the present invention.
- Vb built-in voltage
- the threshold voltage for the second N-channel MOS transistor region including the second gate conductor layer 5 b to which the word line WL is connected decreases due to a substrate bias effect.
- This state is assigned to logical storage data “ 1 ”.
- FIG. 5B a memory block selected before writing is in an erase state “0” in advance, and the voltage V FB of the channel region 7 is equal to V FB “0”. With a write operation, a write state “1” is stored at random.
- logical storage data of logical “0” and that of logical “1” are created for the word line WL.
- the level difference between the two threshold voltages of the word line WL is used to perform reading by a sense amplifier.
- the voltage applied to the first gate conductor layer 5 a connected to the plate line PL is set to a voltage higher than the threshold voltage at the time of logical storage data “1” and lower than the threshold voltage at the time of logical storage data “0” in data reading, a property that a current does not flow even when the voltage of the word line WL is increased can be attained as illustrated in FIG. 5C .
- a split-page write operation of the dynamic flash memory cell according to the first embodiment of the present invention will be described with reference to FIG. 6A to FIG. 6E .
- FIG. 6A illustrates a part of a block (which is an example of “block” in the claims) in which a plurality of memory cells C 00 to C 53 are arranged in a matrix in four rows and six columns. In the actual memory block, the number of memory cells is larger than that of four rows and six columns.
- a corresponding one of the word lines WL 0 to WL 3 a corresponding one of the plate lines PL 0 to PL 3 , and a corresponding one of the bit lines BL 0 to BL 5 are connected.
- the bit lines BL 0 to BL 5 are respectively connected to sense amplifier circuits SA 0 to SA 5 .
- the split-page write operation is an operation in which a selected page for which the page erase operation is performed in advance is split into at least one or more sets of split pages (which is an example of “split pages” in the claims) and a page write operation is performed for all memory cells included in the selected page by performing the split-page write operation at least once.
- FIG. 6B is an operation waveform diagram of the split-page write operation, and the word line WL 1 and the plate line PL 1 connected to the memory cells C01, C11, C21, C31, C41, and C51 are selected.
- the period from time T 1 to time T 4 is assumed to be a first period (which is an example of “first period” in the claims) of the split-page write operation, and the period from time T 5 to time T 8 is assumed to be a second period (which is an example of “second period” in the claims) of the split-page write operation.
- the word line WL 1 rises from a first voltage V 1 to a second voltage V 2 .
- the bit lines BL 0 to BL 5 are at a low voltage of a third voltage V 3 , and therefore, the inversion layer 12 b is formed in the second channel semiconductor layer 7 b.
- the bit lines BL 0 and BL 2 rise from the third voltage V 3 to a fourth voltage V 4 in accordance with data, stored in advance in the sense amplifier circuits SA 0 to SA 2 , for a first group of memory cells (which is an example of “first group of memory cells” in the claims) to be subjected to the split-page write operation.
- the word line WL 1 rises to the second voltage V 2 prior to the bit lines BL 0 and BL 2 , and therefore, a “1” split-page write operation is performed in the memory cells C01 and C21.
- the bit lines BL 0 and BL 2 at the fourth voltage V 4 drop to the third voltage V 3 .
- the word line WL 1 is at a high voltage of the second voltage V 2 .
- the inversion layer 12 b is formed in the second channel semiconductor layer 7 b.
- the word line WL 1 drops from the second voltage V 2 to the first voltage V 1 .
- the inversion layer 12 b is present in the second channel semiconductor layer 7 b, capacitive coupling between the word line WL 1 and the channel semiconductor layer 7 b is interrupted (shielded), and the group of positive holes 9 stored in the channel semiconductor layers of the memory cells C01 and C21 to which “1” is written are prevented from being discharged through either the first impurity layer 3 a or the second impurity layer 3 b or both the first impurity layer 3 a and the second impurity layer 3 b.
- the word line WL 1 rises again from the first voltage V 1 to the second voltage V 2 .
- the bit lines BL 0 to BL 5 are at a low voltage of the third voltage V 3 , and therefore, the inversion layer 12 b is formed in the second channel semiconductor layer 7 b.
- the bit lines BL 3 and BL 4 rise from the third voltage V 3 to the fourth voltage V 4 in accordance with data, stored in advance in the sense amplifier circuits SA 3 to SA 5 , for a second group of memory cells (which is an example of “second group of memory cells” in the claims) to be subjected to the split-page write operation.
- the word line WL 1 rises to the second voltage V 2 prior to the bit lines BL 3 and BL 4 , and therefore, a “1” split-page write operation is performed in the memory cells C31 and C41.
- the bit lines BL 3 and BL 4 at the fourth voltage V 4 drop to the third voltage V 3 .
- the word line WL 1 is at a high voltage of the second voltage V 2 .
- the inversion layer 12 b is formed in the second channel semiconductor layer 7 b.
- the word line WL 1 drops from the second voltage V 2 to the first voltage V 1 .
- the inversion layer 12 b is present in the second channel semiconductor layer 7 b, capacitive coupling between the word line WL 1 and the channel semiconductor layer 7 b is interrupted (shielded), and the group of positive holes 9 stored in the channel semiconductor layers of the memory cells C31 and C41 to which “1” is written are prevented from being discharged through either the first impurity layer 3 a or the second impurity layer 3 b or both the first impurity layer 3 a and the second impurity layer 3 b.
- the source line SL may be kept at a fifth voltage V 5 .
- the plate line PL 1 that is the first driving control line may be kept at a sixth voltage V 6 .
- the plate lines PL 0 to PL 3 may have a structure common to the block and need not be decoded.
- a split-page write operation in which the memory cells C01, C11, C21, C31, C41, and C51 included in the first page and memory cells C02, C12, C22, C32, C42, and C52 included in the second page are alternately selected will be described with reference to the operation waveform diagram in FIG. 6C .
- the period from time S 1 to time S 4 is assumed to be a third period (which is an example of “third period” in the claims) of the split-page write operation, and the period from time S 5 to time S 8 is assumed to be a fourth period (which is an example of “fourth period” in the claims) of the split-page write operation.
- the split-page write operation in the first period from time T 1 to time T 4 in FIG. 6C is the same as in FIG. 6B , and therefore, a description thereof will be omitted.
- the word line WL 2 rises from the first voltage V 1 to the second voltage V 2 at time S 1 .
- the bit lines BL 0 to BL 5 are at a low voltage of the third voltage V 3 , and therefore, the inversion layer 12 b is formed in the second channel semiconductor layer 7 b.
- the bit lines BL 1 and BL 2 rise from the third voltage V 3 to the fourth voltage V 4 in accordance with data for the split-page write operation for a third group of memory cells (which is an example of “third group of memory cells” in the claims) in the third period, the data being different from that in the first period and having being loaded to the sense amplifier circuits SA 0 to SA 2 .
- the word line WL 2 rises to the second voltage V 2 prior to the bit lines BL 1 and BL 2 , and therefore, a “1” split-page write operation is performed in the memory cells C 12 and C 22 .
- the bit lines BL 1 and BL 2 at the fourth voltage V 4 drop to the third voltage V 3 .
- the word line WL 2 is at a high voltage of the second voltage V 2 .
- the inversion layer 12 b is formed in the second channel semiconductor layer 7 b.
- the word line WL 2 drops from the second voltage V 2 to the first voltage V 1 .
- the inversion layer 12 b is present in the second channel semiconductor layer 7 b, capacitive coupling between the word line WL 2 and the channel semiconductor layer 7 b is interrupted (shielded), and the group of positive holes 9 stored in the channel semiconductor layers of the memory cells C12 and C22 to which “1” is written are prevented from being discharged through either the first impurity layer 3 a or the second impurity layer 3 b or both the first impurity layer 3 a and the second impurity layer 3 b.
- the word line WL 1 rises again from the first voltage V 1 to the second voltage V 2 .
- the bit lines BL 0 to BL 5 are at a low voltage of the third voltage V 3 , and therefore, the inversion layer 12 b, is formed in the second channel semiconductor layer 7 b.
- the bit lines BL 3 and BL 4 rise from the third voltage V 3 to the fourth voltage V 4 in accordance with data, for split-page writing in the second period, stored in advance in the sense amplifier circuits SA 3 to SA 5 .
- the word line WL 1 rises to the second voltage V 2 prior to the bit lines BL 3 and BL 4 , and therefore, a “1” split-page write operation is performed in the memory cells C31 and C41.
- the bit lines BL 3 and BL 4 at the fourth voltage V 4 drop to the third voltage V 3 .
- the word line WL 1 is at a high voltage of the second voltage V 2 .
- the inversion layer 12 b is formed in the second channel semiconductor layer 7 b.
- the word line WL 1 drops from the second voltage V 2 to the first voltage V 1 .
- the inversion layer 12 b is present in the second channel semiconductor layer 7 b, capacitive coupling between the word line WL 1 and the channel semiconductor layer 7 b is interrupted (shielded), and the group of positive holes 9 stored in the channel semiconductor layers of the memory cells C31 and C41 to which “1” is written are prevented from being discharged through either the first impurity layer 3 a or the second impurity layer 3 b or both the first impurity layer 3 a and the second impurity layer 3 b.
- the word line WL 2 rises again from the first voltage V 1 to the second voltage V 2 .
- the bit lines BL 0 to BL 5 are at a low voltage of the third voltage V 3 , and therefore, the inversion layer 12 b, is formed in the second channel semiconductor layer 7 b.
- the bit lines BL 3 and BL 5 rise from the third voltage V3 to the fourth voltage V 4 in accordance with data for the split-page write operation for a fourth group of memory cells (which is an example of “fourth group of memory cells” in the claims) in the fourth period, the data being different from that in the second period and having been loaded to the sense amplifier circuits SA 3 to SA 5 .
- the word line WL 2 rises to the second voltage V 2 prior to the bit lines BL 3 and BL 5 , and therefore, a “1” split-page write operation is performed in the memory cells C32 and C52.
- the bit lines BL 3 and BL 5 at the fourth voltage V 4 drop to the third voltage V 3 .
- the word line WL 2 is at a high voltage of the second voltage V 2 .
- the inversion layer 12 b is formed in the second channel semiconductor layer 7 b.
- the word line WL 2 drops from the second voltage V 2 to the first voltage V 1 .
- the inversion layer 12 b is present in the second channel semiconductor layer 7 b, capacitive coupling between the word line WL 2 and the channel semiconductor layer 7 b is interrupted (shielded), and the group of positive holes 9 stored in the channel semiconductor layers of the memory cells C32 and C52 to which “1” is written are prevented from being discharged through either the first impurity layer 3 a or the second impurity layer 3 b or both the first impurity layer 3 a and the second impurity layer 3 b.
- FIG. 6D illustrates an example in which in a state in which the word line WL 1 and the plate line PL 1 of the first page are selected, in the first period, a first group of bit lines BL 0 to BL 2 (which is an example of “first group of bit lines” in the claims) is selected and the split-page write operation of the first group of memory cells to which the first group of bit lines is connected is performed, and in the second period, a second group of bit lines BL 3 to BL 5 (which is an example of “second group of bit lines” in the claims) is selected and the split-page write operation of the second group of memory cells to which the second group of bit lines is connected is performed.
- FIG. 6E illustrates an example in which in split-page writing of the first page, the plate line PL 1 is clock-operated similarly to the word line WL 1 .
- the plate line PL 1 rises from a low voltage of the sixth voltage V 6 to a high voltage of a seventh voltage V 7 that is higher than the sixth voltage V 6 .
- the operations of the dynamic flash memory described in this embodiment can be performed. Further, a dynamic flash memory cell having a round shape, a dynamic flash memory cell having an elliptic shape, and a dynamic flash memory cell having a rectangular shape may coexist on the same chip.
- the dynamic flash memory element including an example SGT in which the first gate insulating layer 4 a and the second gate insulating layer 4 b that surround the entire side surface of the Si pillar 2 standing on the substrate in the vertical direction are provided and which includes the first gate conductor layer 5 a and the second gate conductor layer 5 b that entirely surround the first gate insulating layer 4 a and the second gate insulating layer 4 b has been described.
- the dynamic flash memory element needs to have a structure that satisfies the condition that the group of positive holes 9 generated by an impact ionization phenomenon are retained in the channel region 7 .
- the channel region 7 needs to have a floating body structure isolated from the substrate.
- the dynamic flash memory element may have a device structure using SOI (Silicon On Insulator) (see, for example, NPL 7 to NPL 10).
- SOI Silicon On Insulator
- the channel region has a floating body structure. Accordingly, the dynamic flash memory element provided in this embodiment needs to satisfy the condition that the channel region has a floating body structure. Even with a structure in which a Fin transistor (see, for example, NPL 13) is formed on an SOI substrate, when the channel region has a floating body structure, the dynamic flash operations can be performed.
- a Fin transistor see, for example, NPL 13
- electron-positive hole pairs may be generated by using a gate-induced drain leakage (GIDL) current (see for example, NPL 14), and the channel region 7 may be filled with the generated group of positive holes.
- GIDL gate-induced drain leakage
- Expressions (1) to (12) provided in the specification and in the drawings are expressions used to qualitatively explain the phenomena, and are not intended to limit the phenomena.
- the reset voltages of the word line WL, the bit line BL, and the source line SL are specified as Vss in the explanations of FIGS. 3AA-3AC and FIG. 3B , the reset voltages may be set to different voltages.
- FIGS. 4BA-4BD , FIGS. 4DA-4DD , and FIGS. 4EA-4ED illustrate example conditions of the page erase operation
- the voltages applied to the source line SL, the plate line PL, the bit line BL, and the word line WL may be changed as long as a state in which the group of positive holes 9 in the channel region 7 are discharged through either the N + layer 3 a or the N + layer 3 b or both the N + layer 3 a and the N + layer 3 b can be attained.
- a voltage may be applied to the source line SL of a selected page, and the bit line BL may be put in a floating state.
- a voltage may be applied to the bit line BL of a selected page, and the source line SL may be put in a floating state.
- the potential distribution of the first channel region 7 a and that of the second channel region 7 b are connected and formed. Accordingly, the first channel region 7 a and the second channel region 7 b that constitute the channel region 7 are connected in the vertical direction in the region surrounded by the insulating layer 6 that is the first insulating layer.
- a fixed voltage of, for example, 2 V may be applied in operation modes other than a mode in which selective erasing is performed in a block erase operation.
- the meaning of “cover” in a case of “for example, a gate insulating layer or a gate conductor layer covers, for example, a channel” also includes a case of surrounding entirely as in an SGT or GAA, a case of surrounding except a portion as in a Fin transistor, and a case of overlapping a flat object as in a planar transistor.
- first gate conductor layer 5 a entirely surrounds the first gate insulating layer 4 a in FIG. 1
- a structure may be employed in which the first gate conductor layer 5 a partially surrounds the first gate insulating layer 4 a in plan view.
- the outer side of the first gate insulating layer not covered by the first gate conductor layer 5 a may be covered by an insulating layer or a third gate conductor layer electrically isolated from the first gate conductor layer.
- a constant voltage or a pulse voltage can be applied to the third gate conductor layer to perform the operations of the dynamic flash memory.
- the first gate conductor layer 5 a may be divided into two or more gate conductor layers, and each of the gate conductor layers may function as a conductive electrode of the plate line and surround the channel region 7 a.
- the two or more gate conductor layers obtained as a result of division may be operated synchronously or asynchronously at the same driving voltage or different driving voltages.
- the second gate conductor layer 5 b may be divided into two or more gate conductor layers, and the gate conductor layers may each function as a conductive electrode of the word line and may be operated synchronously or asynchronously at the same driving voltage or different driving voltages.
- the operations of the dynamic flash memory can be performed.
- the first gate conductor layer 5 a is divided into two or more gate conductor layers
- at least one of the first gate conductor layers obtained as a result of division assumes the roles of the first gate conductor layer 5 a described above.
- the second gate conductor layer 5 b is divided into two or more gate conductor layers
- at least one of the second gate conductor layers obtained as a result of division assumes the roles of the second gate conductor layer 5 b described above.
- the first gate conductor layer 5 a may be connected to the word line WL, and the second gate conductor layer 5 b may be connected to the plate line PL. Also in this case, the operations of the dynamic flash memory described above can be performed.
- the above-described conditions of voltages applied to the bit line BL, the source line SL, the word line WL, and the plate line PL and the voltage of the floating body are examples for performing basic operations including the erase operation, the write operation, and the read operation, and other voltage conditions may be employed as long as basic operations of the present invention can be performed.
- This embodiment has the following features.
- the dynamic flash memory cell of this embodiment is constituted by the N + layers 3 a and 3 b that function as the source and the drain, the channel region 7 , the first gate insulating layer 4 a, the second gate insulating layer 4 b, the first gate conductor layer 5 a, and the second gate conductor layer 5 b, which are formed in a pillar form as a whole.
- the N + layer 3 a that functions as the source is connected to the source line SL
- the N + layer 3 b that functions as the drain is connected to the bit line BL
- the first gate conductor layer 5 a is connected to the plate line PL
- the second gate conductor layer 5 b is connected to the word line WL.
- a structure is employed in which the gate capacitance of the first gate conductor layer 5 a to which the plate line PL is connected is larger than the gate capacitance of the second gate conductor layer 5 b to which the word line WL is connected, which is a feature.
- the first gate conductor layer and the second gate conductor layer are stacked in the vertical direction. Accordingly, even when the structure is employed in which the gate capacitance of the first gate conductor layer 5 a to which the plate line PL is connected is larger than the gate capacitance of the second gate conductor layer 5 b to which the word line WL is connected, the memory cell area does not increase in plan view. Accordingly, a high-performance and highly integrated dynamic flash memory cell can be implemented.
- the user can select a data size for the split-page write operation as desired within the range of the number of memory cells (page size) connected to the page.
- the group of memory cells for the split-page write operation can be easily split, a selected page is changed among a plurality of different pages, and the split-page write operation can be performed for different groups of memory cells.
- the DRAM that is a nonvolatile memory in the related art is a read-destructive-type memory, and therefore, writing needs to be performed simultaneously for all memory cells in a selected page also in a page write operation.
- the split-page write operation of the dynamic flash memory cell of the present invention the system flexibility is significantly increased for users who handle various data capacities. As a result, a high-speed system can be attained.
- the word line WL rises prior to the bit line BL.
- the inversion layer 12 b is formed in the channel semiconductor layer 7 of the memory cell to which “1” is written, to interrupt (shield) capacitive coupling between the word line WL and the channel semiconductor layer 7 . Therefore, some of the group of positive holes 9 stored in the channel semiconductor layer 7 are not discharged, and a highly reliable memory device can be provided.
- the voltage of the word line WL changes.
- the plate line PL assumes the role of decreasing the capacitive coupling ratio between the word line WL and the channel region 7 .
- an effect on changes in the voltage of the channel region 7 when the voltage of the word line WL changes can be substantially suppressed.
- the difference between the threshold voltages for the transistor regions of the word line WL indicating logical “0” and logical “1” can be increased. This leads to an increased operation margin of the dynamic flash memory cell.
- the plate line common to the memory cells C00 to C53 may be used.
- the processes and circuits can be more simplified, and a higher speed can be attained.
- the Si pillar is formed in the present invention
- the Si pillar may be a semiconductor pillar made of a semiconductor material other than Si. This is similarly applicable to other embodiments according to the present invention.
- vertical NAND-type flash memory circuits memory cells that are stacked in a plurality of stages in the vertical direction and each of which is constituted by a semiconductor pillar, which functions as the channel, and a tunnel oxide layer, a charge storage layer, an interlayer insulating layer, and a control conductor layer that surround the semiconductor pillar are formed.
- a source line impurity layer corresponding to the source and a bit line impurity layer corresponding to the drain are disposed respectively.
- the vertical NAND-type flash memory circuit is one type of SGT circuit. Therefore, the present invention is also applicable to a circuit in which a NAND-type flash memory circuit coexists.
- electron-positive hole pairs may be generated by an impact ionization phenomenon using a gate-induced drain leakage (GIDL) current described in NPL 10 and NPL 14, and the floating body FB may be filled with the generated group of positive holes.
- GIDL gate-induced drain leakage
- the operations of the dynamic flash memory can be performed.
- the Si pillar 2 that is of N-type the majority carriers are electrons. Therefore, a group of electrons generated by impact ionization are stored in the channel region 7 , and a “1” state is set.
- a high-density and high-performance dynamic flash memory that is a memory device including an SGT can be obtained.
Abstract
Description
- This application claims priority claims priority under 35 U.S.C. §119(a) to PCT/JP2021/015275 filed on Apr. 13, 2021, the entire content of which is incorporated herein by reference.
- The present invention relates to a semiconductor memory device including a semiconductor element.
- Recently, there has been a demand for highly integrated and high-performance memory elements in the development of LSI (Large Scale Integration) technology.
- Typical planar MOS transistors include a channel that extends in a horizontal direction along the upper surface of the semiconductor substrate. In contrast, SGTs include a channel that extends in a direction perpendicular to the upper surface of the semiconductor substrate (see, for example,
PTL 1 and NPL 1). Accordingly, the density of semiconductor devices can be made higher with SGTs than with planar MOS transistors. Such SGTs can be used as selection transistors to implement highly integrated memories, such as a DRAM (Dynamic Random Access Memory, see, for example, NPL 2) to which a capacitor is connected, a PCM (Phase Change Memory, see, for example, NPL 3) to which a resistance change element is connected, an RRAM (Resistive Random Access Memory, see, for example, NPL 4), and an MRAM (Magneto-resistive Random Access Memory, see, for example, NPL 5) that changes the resistance by changing the orientation of a magnetic spin with a current. Further, there exists, for example, a DRAM memory cell (see, for example, NPL 6) constituted by a single MOS transistor and including no capacitor. The present application relates to a dynamic flash memory that can be constituted only by a MOS transistor and that includes no resistance change element or capacitor. -
FIGS. 7A-7D illustrate a write operation of a DRAM memory cell constituted by a single MOS transistor and including no capacitor described above,FIGS. 8A and 8B illustrate a problem in the operation, andFIGS. 9A-9C illustrate a read operation (seeNPL 7 to NPL 10).FIG. 7A illustrates a “1” write state. Here, the memory cell is formed on anSOI substrate 100, is constituted by a source N+ layer 103 (hereinafter, a semiconductor region that contains a donor impurity in high concentrations is referred to as “N+ layer”) to which a source line SL is connected, a drain N+ layer 104 to which a bit line BL is connected, agate conductor layer 105 to which a word line WL is connected, and afloating body 102 of aMOS transistor 110, and includes no capacitor. Thesingle MOS transistor 110 constitutes the DRAM memory cell. Directly under thefloating body 102, a SiO2 layer 101 of the SOI substrate is in contact with thefloating body 102. To write “1” to the memory cell constituted by thesingle MOS transistor 110, theMOS transistor 110 is operated in the saturation region. That is, achannel 107, for electrons, extending from the source N+ layer 103 has a pinch-offpoint 108 and does not reach the drain N+ layer 104 to which the bit line is connected. When a high voltage is applied to both the bit line BL connected to the drain N+ layer and the word line WL connected to thegate conductor layer 105, and theMOS transistor 110 is operated at the gate voltage that is about one-half of the drain voltage, the electric field intensity becomes maximum at the pinch-offpoint 108 that is in the vicinity of the drain N+ layer 104. As a result, accelerated electrons that flow from the source N+ layer 103 toward the drain N+ layer 104 collide with the Si lattice, and with kinetic energy lost at the time of collision, electron-positive hole pairs are generated (impact ionization phenomenon). Most of the generated electrons (not illustrated) reach the drain N+ layer 104. Further, a very small proportion of the electrons that are very hot jump over agate oxide film 109 and reach thegate conductor layer 105. Simultaneously,positive holes 106 are generated with which thefloating body 102 is charged. In this case, the generated positive holes contribute to an increase in the majority carriers because thefloating body 102 is P-type Si. When thefloating body 102 is filled with the generatedpositive holes 106 and the voltage of thefloating body 102 becomes higher than that of the source N+ layer 103 by Vb or more, further generated positive holes are discharged to the source N+ layer 103. Here, Vb is the built-in voltage of the PN junction between the source N+ layer 103 and the P layer, namely, thefloating body 102, and is equal to about 0.7 V.FIG. 7B illustrates a state in which thefloating body 102 is charged to saturation with the generatedpositive holes 106. - Now, a “0” write operation of the
memory cell 110 will be described with reference toFIG. 7C . For the common selection word line WL, thememory cell 110 to which “1” is written and thememory cell 110 to which “0” is written are present at random.FIG. 7C illustrates a state of rewriting from the “1” write state to a “0” write state. To write “0”, the voltage of the bit line BL is set to a negative bias, and the PN junction between the drain N+ layer 104 and the P layer, namely, thefloating body 102, is forward biased. As a result, thepositive holes 106 in thefloating body 102 generated in advance in the previous cycle flow into the drain N+ layer 104 that is connected to the bit line BL. When the write operation ends, the two memory cells are in a state in which the memory cell 110 (FIG. 7B ) is filled with the generatedpositive holes 106, and from the memory cell 110 (FIG. 7C ), the generated positive holes are discharged. The potential of thefloating body 102 of thememory cell 110 filled with thepositive holes 106 becomes higher than that of thefloating body 102 in which generated positive holes are not present. Therefore, the threshold voltage for thememory cell 110 to which “1” is written becomes lower than the threshold voltage for thememory cell 110 to which “0” is written. This is illustrated inFIG. 7D . - Now, a problem in the operation of the memory cell constituted by the
single MOS transistor 110 will be described with reference toFIGS. 8A and 8B . As illustrated inFIG. 8A , the capacitance CFB of the floating body is equal to the sum of the capacitance CWL between the gate to which the word line is connected and the floating body, the junction capacitance CSL of the PN junction between the source N+ layer 103 to which the source line is connected and thefloating body 102, and the junction capacitance CBL of the PN junction between the drain N+ layer 104 to which the bit line is connected and thefloating body 102 and is expressed as follows. -
C FB =C WL +C BL +C SL (10) - The capacitive coupling ratio βWL between the gate to which the word line is connected and the floating body is expressed as follows.
-
βWL =C WL/(C WL +C BL +C SL) (11) - Therefore, a change in the word line voltage VWL at the time of reading or writing affects the voltage of the
floating body 102 that functions as a storage node (contact point) of the memory cell. This is illustrated inFIG. 8B . When the word line voltage VWL rises from 0 V to VLH at the time of reading or writing, the voltage VFB of thefloating body 102 rises from VFB1, which is the voltage in the initial state before the word line voltage changes, to VFB2 due to capacitive coupling with the word line. The voltage change amount ΔVFB is expressed as follows. -
- Here, for βWL in expression (11), contribution of CWL is large and, for example, CWL:CBL:CSL=8:1:1 holds. This results in βWL=0.8. When the word line changes, for example, from 5 V at the time of writing to 0 V after the end of writing, the
floating body 102 receives an amplitude noise of 5 V×βWL=4 V due to capacitive coupling between the word line WL and thefloating body 102. Accordingly, a sufficient margin is not provided to the potential difference between the “1” potential and the “0” potential of thefloating body 102 at the time of writing, which is a problem. -
FIGS. 9A -9C illustrate a read operation whereFIG. 9(a) illustrates a “1” write state andFIG. 9B illustrates a “0” write state. In actuality, however, even when Vb is set for the floatingbody 102 to write “1”, once the word line returns to 0 V at the end of writing, the floatingbody 102 is lowered to a negative bias. When “0” is written, the floatingbody 102 is lowered to a further negative bias, and it is difficult to provide a sufficiently large margin to the potential difference between “1” and “0” at the time of writing as illustrated inFIG. 9C . Therefore, there has been difficulty in commercially introducing DRAM memory cells actually including no capacitor. - [PTL 1] Japanese Unexamined Patent Application Publication No. 2-188966
- [PTL 2] Japanese Unexamined Patent Application Publication No. 3-171768
- [PTL 3] Japanese Patent No. 3957774
- [NPL 1] Hiroshi Takato, Kazumasa Sunouchi, Naoko Okabe, Akihiro Nitayama, Katsuhiko Hieda, Fumio Horiguchi, and Fujio Masuoka: IEEE Transaction on Electron Devices, Vol. 38, No. 3, pp. 573-578 (1991)
- [NPL 2] H. Chung, H. Kim, H. Kim, K. Kim, S. Kim, K. Dong, J. Kim, Y.C. Oh, Y. Hwang, H. Hong, G. Jin, and C. Chung: “4F2 DRAM Cell with Vertical Pillar Transistor (VPT)”, 2011 Proceeding of the European Solid-State Device Research Conference, (2011)
- [NPL 3] H. S. Philip Wong, S. Raoux, S. Kim, Jiale Liang, J. R. Reifenberg, B. Rajendran, M. Asheghi and K. E. Goodson: “Phase Change Memory”, Proceeding of IEEE, Vol. 98, No. 12, December, pp. 2201-2227 (2010)
- [NPL 4] T. Tsunoda, K. Kinoshita, H. Noshiro, Y. Yamazaki, T. Iizuka, Y. Ito, A. Takahashi, A. Okano, Y. Sato, T. Fukano, M. Aoki, and Y. Sugiyama : “Low Power and High Speed Switching of Ti-doped NiO ReRAM under the Unipolar Voltage Source of less than 3V”, IEDM (2007)
- [NPL 5] W. Kang, L. Zhang, J. Klein, Y. Zhang, D. Ravelosona, and W. Zhao: “Reconfigurable Codesign of STT-MRAM Under Process Variations in Deeply Scaled Technology”, IEEE Transaction on Electron Devices, pp. 1-9 (2015)
- [NPL 6] M. G. Ertosum, K. Lim, C. Park, J. Oh, P. Kirsch, and K. C. Saraswat: “Novel Capacitorless Single-Transistor Charge-Trap DRAM (1T CT DRAM) Utilizing Electron”, IEEE Electron Device Letter, Vol. 31, No. 5, pp. 405-407 (2010)
- [NPL 7] J. Wan, L. Rojer, A. Zaslaysky, and S. Critoloveanu: “A Compact Capacitor-Less High-Speed DRAM Using Field Effect-Controlled Charge Regeneration”, Electron Device Letters, Vol. 35, No. 2, pp. 179-181 (2012)
- [NPL 8] T. Ohsawa, K. Fujita, T. Higashi, Y. Iwata, T. Kajiyama, Y. Asao, and K. Sunouchi: “Memory design using a one-transistor gain cell on SOI”, IEEE JSSC, vol. 37, No. 11, pp. 1510-1522 (2002).
- [NPL 9] T. Shino, N. Kusunoki, T. Higashi, T. Ohsawa, K. Fujita, K. Hatsuda, N. Ikumi, F. Matsuoka, Y. Kajitani, R. Fukuda, Y. Watanabe, Y. Minami, A. Sakamoto, J. Nishimura, H. Nakajima, M. Morikado, K. Inoh, T. Hamamoto, A. Nitayama: “Floating Body RAM Technology and its Scalability to 32nm Node and Beyond”, IEEE IEDM (2006).
- [NPL 10] E. Yoshida: “A Capacitorless 1T-DRAM Technology Using Gate-Induced Drain-Leakage (GIDL) Current for Low-Power and High-Speed Embedded Memory”, IEEE IEDM (2006).
- [NPL 11] J. Y. Song, W. Y. Choi, J. H. Park, J. D. Lee, and B-G. Park: “Design Optimization of Gate-All-Around (GAA) MOSFETs”, IEEE Trans. Electron Devices, vol. 5, no. 3, pp. 186-191, May 2006.
- [NPL 12] N. Loubet, et al.: “Stacked Nanosheet Gate-All-Around Transistor to Enable Scaling Beyond FinFET”, 2017 IEEE Symposium on VLSI Technology Digest of Technical Papers, T17-5, T230-T231, June 2017.
- [NPL 13] H. Jiang, N. Xu, B. Chen, L. Zengl, Y. He, G. Du, X. Liu and X. Zhang: “Experimental investigation of self heating effect (SHE) in multiple-fin SOI FinFETs”, Semicond. Sci. Technol. 29 (2014) 115021 (7pp).
- [NPL 14] E. Yoshida, and T. Tanaka: “A Capacitorless 1T-DRAM Technology Using Gate-Induced Drain-Leakage (GIDL) Current for Low-Power and High-Speed Embedded Memory”, IEEE Transactions on Electron Devices, Vol. 53, No. 4, pp. 692-697, Apr. 2006.
- In capacitor-less single-transistor DRAMs (gain cells), capacitive coupling between the word line and the floating body is strong. When the potential of the word line is changed at the time of data reading or at the time of data writing, the change is transmitted as direct noise to the floating body, which is a problem. This causes a problem of erroneous reading or erroneous rewriting of storage data and makes it difficult to commercially introduce capacitor-less single-transistor DRAMs (gain cells).
- To solve the above-described problem, a semiconductor element memory device according to the present invention is
- a memory device including a plurality of pages arranged in a column direction, each of the pages being constituted by a plurality of memory cells arranged in a row direction on a substrate,
- each memory cell included in each of the pages including:
- a semiconductor base material that stands on the substrate in a vertical direction or that extends in a horizontal direction along the substrate;
- a first impurity layer and a second impurity layer that are disposed at respective ends of the semiconductor base material;
- a first gate insulating layer that partially or entirely surrounds a side surface of the semiconductor base material between the first impurity layer and the second impurity layer and that is in contact with or in close vicinity to the first impurity layer;
- a second gate insulating layer that surrounds the side surface of the semiconductor base material, that is connected to the first gate insulating layer, and that is in contact with or in close vicinity to the second impurity layer;
- a first gate conductor layer that partially or entirely covers the first gate insulating layer;
- a second gate conductor layer that covers the second gate insulating layer; and
- a channel semiconductor layer that is the semiconductor base material and that is covered by the first gate insulating layer and the second gate insulating layer, in which
- voltages applied to the first gate conductor layer, the second gate conductor layer, the first impurity layer, and the second impurity layer are controlled to perform a page write operation and a page erase operation, and
- all memory cells included in a first page subjected to the page erase operation perform the page write operation at least once (first invention).
- In the first invention described above, the first page is constituted by at least one or more sets of split pages, a first group of memory cells included in the first page performs a split-page write operation in a first period, and a second group of memory cells included in the first page performs the split-page write operation in a second period (second invention).
- In the second invention described above, a third group of memory cells included in a second page among the plurality of pages performs the split-page write operation in a third period, a fourth group of memory cells included in the second page performs the split-page write operation in a fourth period, and the first period is followed by the third period, the second period, and the fourth period in a time-series order (third invention).
- A semiconductor element memory device includes a block in which the plurality of pages of the memory device according to any of the first to third inventions described above are arranged, in which
- the first impurity layer of the memory cell included in the plurality of pages is connected to a source line, the second impurity layer thereof is connected to a bit line, one of the first gate conductor layer or the second gate conductor layer thereof is connected to a word line, and the other of the first gate conductor layer or the second gate conductor layer is connected to a first driving control line,
- the source line is connected to the semiconductor base materials in the block, and
- voltages are applied to the source line, the bit line, the first driving control line, and the word line to perform the page erase operation of discharging a group of positive holes in all the semiconductor base materials in the selected page in the block (fourth invention).
- In the fourth invention described above, in the page write operation, in a state in which the first page is selected, in the first period, a first group of bit lines is selected and the split-page write operation of the first group of memory cells to which the first group of bit lines is connected is performed, and in the second period, a second group of bit lines is selected, and the split-page write operation of the second group of memory cells to which the second group of bit lines is connected is performed (fifth invention).
- In the first invention described above, a first gate capacitance between the first gate conductor layer and the channel semiconductor layer is larger than a second gate capacitance between the second gate conductor layer and the channel semiconductor layer (sixth invention).
- In the first invention described above, the first gate conductor layer is isolated into two conductor layers around the first gate insulating layer in plan view (seventh invention).
- In the first invention described above, in the page write operation, a group of positive holes generated by an impact ionization phenomenon are retained inside the channel semiconductor layer, and a voltage of the channel semiconductor layer is made equal to a first data retention voltage that is higher than the voltage of either the first impurity layer or the second impurity layer or that is higher than the voltages of both the first impurity layer and the second impurity layer, and
- in the page erase operation, the voltages applied to the first impurity layer, the second impurity layer, the first gate conductor layer, and the second gate conductor layer are controlled to discharge the group of positive holes through either the first impurity layer or the second impurity layer or both the first impurity layer and the second impurity layer, and the voltage of the channel semiconductor layer is made equal to a second data retention voltage that is lower than the first data retention voltage (eighth invention).
-
FIG. 1 is a structural diagram of a memory device including an SGT according to a first embodiment. -
FIGS. 2A, 2B and 2C are diagrams for explaining an effect attained in a case where the gate capacitance of a firstgate conductor layer 5 a connected to a plate line PL is made larger than the gate capacitance of a secondgate conductor layer 5 b to which a word line WL is connected in the memory device including an SGT according to the first embodiment. -
FIGS. 3AA, 3AB , and 3ACare diagrams for explaining a mechanism of a write operation of the memory device including an SGT according to the first embodiment. -
FIG. 3B are diagrams for explaining the mechanism of the write operation of the memory device including an SGT according to the first embodiment. -
FIG. 4A is a diagram for explaining a mechanism of a page erase operation of the memory device including an SGT according to the first embodiment. -
FIGS. 4BA, 4BB, 4BC and 4BD are diagrams for explaining the mechanism of the page erase operation of the memory device including an SGT according to the first embodiment. -
FIG. 4C are diagrams for explaining the mechanism of the page erase operation of the memory device including an SGT according to the first embodiment. -
FIGS. 4DA, 4DB, 4DC and 4DD are diagrams for explaining a mechanism of the page erase operation of the memory device including an SGT according to the first embodiment. -
FIGS. 4EA, 4EB, 4EC and 4ED are diagrams for explaining a mechanism of the page erase operation of the memory device including an SGT according to the first embodiment. -
FIGS. 5A, 5B and 5C are diagrams for explaining a mechanism of a read operation of the memory device including an SGT according to the first embodiment. -
FIG. 6A is a circuit block diagram for explaining a split-page write operation of the memory device including an SGT according to the first embodiment. -
FIG. 6B is an operation waveform diagram for explaining the split-page write operation of the memory device including an SGT according to the first embodiment. -
FIG. 6C is a diagram for explaining the split-page write operation of the memory device including an SGT according to the first embodiment. -
FIG. 6D is a diagram for explaining the split-page write operation of the memory device including an SGT according to the first embodiment. -
FIG. 6E is a diagram for explaining the split-page write operation of the memory device including an SGT according to the first embodiment. -
FIGS. 7A, 7B, 7C and 7D are diagrams for explaining a write operation of a DRAM memory cell including no capacitor in the related art. -
FIGS. 8A and 8B are diagrams for explaining a problem in the operation of the DRAM memory cell including no capacitor in the related art. -
FIGS. 9A, 9B and 9C are diagrams for explaining a read operation of the DRAM memory cell including no capacitor in the related art. - Hereinafter, embodiments of a memory device (hereinafter called a dynamic flash memory) including a semiconductor element according to the present invention will be described with reference to the drawings.
- The structure and operation mechanisms of a dynamic flash memory cell according to a first embodiment of the present invention will be described with reference to
FIG. 1 toFIGS. 5A-5C . The structure of the dynamic flash memory cell will be described with reference toFIG. 1 . An effect attained in a case where the gate capacitance of a firstgate conductor layer 5 a connected to a plate line PL is made larger than the gate capacitance of a secondgate conductor layer 5 b to which a word line WL is connected will be described with reference toFIGS. 2A-2C . A mechanism of a data write operation will be described with reference toFIGS. 3AA-3AC andFIG. 3B , mechanisms of a data erase operation will be described with reference toFIGS. 4A toFIGS. 4EA-4ED , and a mechanism of a data read operation will be described with reference toFIGS. 5A-5C . -
FIG. 1 illustrates the structure of the dynamic flash memory cell according to the first embodiment of the present invention. On the top and the bottom of a silicon semiconductor pillar 2 (hereinafter, the silicon semiconductor pillar is referred to as “Si pillar”) (which is an example of “semiconductor base material” in the claims) of the P or i (intrinsic) conductivity type formed on a substrate, N+ layers 3 a and 3 b (which are examples of “first impurity layer” and “second impurity layer” in the claims), one of which functions as the source and the other functions as the drain, are formed respectively. The part of theSi pillar 2 between the N+ layers 3 a and 3 b that function as the source and the drain functions as a channel region 7 (which is an example of “channel semiconductor layer” in the claims). Around thechannel region 7, a firstgate insulating layer 4 a (which is an example of “first gate insulating layer” in the claims) and a secondgate insulating layer 4 b (which is an example of “second gate insulating layer” in the claims) are formed. The firstgate insulating layer 4 a and the secondgate insulating layer 4 b are in contact with or in close vicinity to the N+ layers 3 a and 3 b that function as the source and the drain respectively. Around the firstgate insulating layer 4 a and the secondgate insulating layer 4 b, the firstgate conductor layer 5 a (which is an example of “first gate conductor layer” in the claims) and the secondgate conductor layer 5 b (which is an example of “second gate conductor layer” in the claims) are formed respectively. The firstgate conductor layer 5 a and the secondgate conductor layer 5 b are isolated from each other by an insulating layer 6 (which is also referred to as “first insulating layer”). Thechannel region 7 between the N+ layers 3 a and 3b is constituted by a first channel Si layer 7 a (which is also referred to as “first channel semiconductor layer”) surrounded by the firstgate insulating layer 4 a and a secondchannel Si layer 7 b (which is also referred to as “second channel semiconductor layer”) surrounded by the secondgate insulating layer 4 b. Accordingly, the N+ layers 3 a and 3b that function as the source and the drain, thechannel region 7, the firstgate insulating layer 4 a, the secondgate insulating layer 4 b, the firstgate conductor layer 5 a, and the secondgate conductor layer 5 b constitute a dynamicflash memory cell 10. The N+ layer 3 a that functions as the source is connected to a source line SL (which is an example of “source line” in the claims), the N+ layer 3 b that functions as the drain is connected to a bit line BL (which is an example of “bit line” in the claims), the firstgate conductor layer 5 a is connected to the plate line PL (which is an example of “first driving control line” in the claims), and the secondgate conductor layer 5 b is connected to the word line WL (which is an example of “word line” in the claims). Desirably, the structure is such that the gate capacitance of the firstgate conductor layer 5 a to which the plate line PL is connected is larger than the gate capacitance of the secondgate conductor layer 5 b to which the word line WL is connected. - In
FIG. 1 , to make the gate capacitance of the firstgate conductor layer 5 a connected to the plate line PL larger than the gate capacitance of the secondgate conductor layer 5 b to which the word line WL is connected, the gate length of the firstgate conductor layer 5 a is made longer than the gate length of the secondgate conductor layer 5 b. Alternatively, instead of making the gate length of the firstgate conductor layer 5 a longer than the gate length of the secondgate conductor layer 5 b, the thicknesses of the respective gate insulating layers may be made different such that the thickness of the gate insulating film of the firstgate insulating layer 4 a is thinner than the thickness of the gate insulating film of the secondgate insulating layer 4 b. Alternatively, the dielectric constants of the materials of the respective gate insulating layers may be made different such that the dielectric constant of the gate insulating film of the firstgate insulating layer 4 a is higher than the dielectric constant of the gate insulating film of the secondgate insulating layer 4 b. The gate capacitance of the firstgate conductor layer 5 a connected to the plate line PL may be made larger than the gate capacitance of the secondgate conductor layer 5 b to which the word line WL is connected, by a combination of any of the lengths of the gate conductor layers 5 a and 5 b and the thicknesses and dielectric constants of thegate insulating layers -
FIGS. 2A-2C are diagrams for explaining an effect attained in a case where the gate capacitance of the firstgate conductor layer 5 a connected to the plate line PL is made larger than the gate capacitance of the secondgate conductor layer 5 b to which the word line WL is connected. -
FIG. 2A is a simplified structural diagram of the dynamic flash memory cell according to the first embodiment of the present invention and illustrates only main parts. To the dynamic flash memory cell, the bit line BL, the word line WL, the plate line PL, and the source line SL are connected, and the potential state of thechannel region 7 is determined by the voltage states of the lines. -
FIG. 2B is a diagram for explaining the capacitance relationships of the respective lines. The capacitance CFB of thechannel region 7 is equal to the sum of the capacitance CWL between thegate conductor layer 5 b to which the word line WL is connected and thechannel region 7, the capacitance CPL between thegate conductor layer 5 a to which the plate line PL is connected and thechannel region 7, the junction capacitance CSL of the PN junction between the source N+ layer 3 a to which the source line SL is connected and thechannel region 7, and the junction capacitance CBL of the PN junction between the drain N+ layer 3 b to which the bit line BL is connected and thechannel region 7, and is expressed as follows. -
C FB =C WL +C PL +C BL +C SL (1) - Therefore, the coupling ratio βWL between the word line WL and the
channel region 7, the coupling ratio βPL between the plate line PL and thechannel region 7, the coupling ratio βBL between the bit line BL and thechannel region 7, and the coupling ratio βSL between the source line SL and thechannel region 7 are expressed as follows. -
βWL =C WL/(C WL +C PL +C BL +C SL) (2) -
βPL =C PL/(C WL +C PL +C BL +C SL) (3) -
βBL =C BL/(C WL +C PL +C BL +C SL) (4) -
βSL =C SL/(C WL +C PL +C BL +C SL) (5) - Here, CpL >CWL holds, and therefore, this results in βPL>βOWL.
-
FIG. 2C is a diagram for explaining a change in the voltage VFB of thechannel region 7 when the voltage VWL of the word line WL rises at the time of a read operation or a write operation and subsequently drops. Here, the potential difference ΔVFB when the voltage VWL of the word line WL rises from 0 V to a high voltage state VWLH and the voltage VFB of thechannel region 7 transitions from a low voltage state VFBL to a high voltage state VFBH is expressed as follows. -
- The coupling ratio βWL between the word line WL and the
channel region 7 is small and the coupling ratio βPL between the plate line PL and thechannel region 7 is large, and therefore, ΔVFB is small, and the voltage VFB of thechannel region 7 negligibly changes even when the voltage VWL of the word line WL changes at the time of a read operation or a write operation. -
FIGS. 3AA-3AC andFIG. 3B illustrate a memory write operation (which is an example of “page write operation” in the claims) of the dynamic flash memory cell according to the first embodiment of the present invention.FIG. 3AA illustrates a mechanism of the write operation, andFIG. 3AB illustrates operation waveforms of the bit line BL, the source line SL, the plate line PL, the word line WL, and thechannel region 7 that functions as a floating body FB. At time T0, the dynamic flash memory cell is in a “0” erase state, and the voltage of thechannel region 7 is equal to VFB“0”. Vss is applied to the bit line BL, the source line SL, and the word line WL, and VBLL is applied to the plate line PL. Here, for example, Vss is equal to 0 V and VPLL is equal to 2 V. Subsequently, from time T1 to time T2, when the bit line BL rises from Vss to VBLH, in a case where, for example, Vss is equal to 0 V, the voltage of thechannel region 7 becomes equal to VFB“0”+βBL×VBLH due to capacitive coupling between the bit line BL and thechannel region 7. - The description of the write operation of the dynamic flash memory cell will be continued with reference to
FIGS. 3AA and 3AB . From time T3 to time T4, the word line WL rises from Vss to VvgLli. Accordingly, when the threshold voltage for “0” erase for a second N-channel MOS transistor region that is a region in which the secondgate conductor layer 5 b to which the word line WL is connected surrounds thechannel region 7 b is denoted by VtWL“0”, as the voltage of the word line WL rises, in a range from Vss to VtWL“0”, the voltage of thechannel region 7 becomes equal to VFB“0”+βBL×VBLH+βWL×VtWL“0” due to second capacitive coupling between the word line WL and thechannel region 7. When the voltage of the word line WL rises to VtWL“0” or above, aninversion layer 12 b in a ring form is formed in thechannel region 7 on the inner periphery of the secondgate conductor layer 5 b and interrupts the second capacitive coupling between the word line WL and thechannel region 7. - The description of the write operation of the dynamic flash memory cell will be continued with reference to
FIGS. 3AA and 3AB . From time T3 to time T4, for example, a fixed voltage VPLL=2 V is applied to the firstgate conductor layer 5 a to which the plate line PL is connected, and the secondgate conductor layer 5 b to which the word line WL is connected is increased to, for example, VWLH=4 V. As a result, as illustrated inFIG. 3AA , aninversion layer 12 a in a ring form is formed in thechannel region 7 on the inner periphery of the firstgate conductor layer 5 a to which the plate line PL is connected, and a pinch-off point 13 is present in theinversion layer 12 a. As a result, a first N-channel MOS transistor region formed of the channel region 7 a that is surrounded by the firstgate conductor layer 5 a operates in the saturation region. In contrast, the second N-channel MOS transistor region including the secondgate conductor layer 5 b to which the word line WL is connected operates in the linear region. As a result, a pinch-off point is not present in thechannel region 7 on the inner periphery of the secondgate conductor layer 5 b to which the word line WL is connected, and theinversion layer 12 b is formed on the entire inner periphery of thegate conductor layer 5 b. Theinversion layer 12 b that is formed on the entire inner periphery of the secondgate conductor layer 5 b to which the word line WL is connected substantially functions as the drain of the first N-channel MOS transistor region. As a result, the electric field becomes maximum in a first boundary region of thechannel region 7 between the first N-channel MOS transistor region including the firstgate conductor layer 5 a and the second N-channel MOS transistor region including the secondgate conductor layer 5 b that are connected in series, and an impact ionization phenomenon occurs in this region. This region is a source-side region when viewed from the second N-channel MOS transistor region including the secondgate conductor layer 5 b to which the word line WL is connected, and therefore, this phenomenon is called a source-side impact ionization phenomenon. By this source-side impact ionization phenomenon, electrons flow from the N+ layer 3 a to which the source line SL is connected toward the N+ layer 3b to which the bit line is connected. The accelerated electrons collide with lattice Si atoms, and electron-positive hole pairs are generated by the kinetic energy. Although some of the generated electrons flow into the firstgate conductor layer 5 a and into the secondgate conductor layer 5 b, most of the generated electrons flow into the N+ layer 3 b to which the bit line BL is connected (not illustrated). - As illustrated in
FIG. 3AC , a generated group of positive holes 9 (which is an example of “group of positive holes” in the claims) are majority carriers in thechannel region 7, with which thechannel region 7 is charged to a positive bias. The N+ layer 3 a to which the source line SL is connected is at 0 V, and therefore, thechannel region 7 is charged up to the built-in voltage Vb (about 0.7 V) of the PN junction between the N+ layer 3 a to which the source line SL is connected and thechannel region 7. When thechannel region 7 is charged to a positive bias, the threshold voltages for the first N-channel MOS transistor region and the second N-channel MOS transistor region decrease due to a substrate bias effect. - The description of the write operation of the dynamic flash memory cell will be continued with reference to
FIG. 3AB . From time T6 to time T7, the voltage of the word line WL drops from VWLH to Vss. During this period, although the second capacitive coupling is formed between the word line WL and thechannel region 7, theinversion layer 12 b interrupts the second capacitive coupling until the voltage of the word line WL drops from VWLH to a threshold voltage VtWL“1” for the second N-channel MOS transistor region or below when the voltage of thechannel region 7 is equal to Vb. Therefore, the capacitive coupling between the word line WL and thechannel region 7 is substantially formed only during a period from when the word line WL drops to VtWL“1” or below to when the word line WL drops to Vss. As a result, the voltage of thechannel region 7 becomes equal to Vb−βWL×VtWL“1”. Here, VtWL“1” is lower than VtWL“0” described above, and βWL×VtWL“1” is small. - The description of the write operation of the dynamic flash memory cell will be continued with reference to
FIG. 3AB . From time T8 to time T9, the bit line BL drops from VBLH to Vss. The bit line BL and thechannel region 7 are capacitively coupled with each other, and therefore, the “1” write voltage VFB“1” of thechannel region 7 becomes as follows at the end. -
V FB“1”=Vb−β WL ×Vt WL“1”−βBL ×V BLH (7) - Here, the coupling ratio βBL between the bit line BL and the
channel region 7 is also small. Accordingly, as illustrated inFIG. 3B , the threshold voltage for the second N-channel MOS transistor region of thesecond channel region 7 b to which the word line WL is connected decreases. The memory write operation in which the voltage VFB“1” in the “1” write state of thechannel region 7 is assumed to be a first data retention voltage (which is an example of “first data retention voltage” in the claims) is performed to assign logical storage data “1”. - At the time of the write operation, electron-positive hole pairs may be generated by an impact ionization phenomenon in a second boundary region between the
first impurity layer 3 a and the first channel semiconductor layer 7 a or in a third boundary region between thesecond impurity layer 3 b and the secondchannel semiconductor layer 7 b instead of the first boundary region, and thechannel region 7 may be charged with the generated group ofpositive holes 9. - A mechanism of a memory erase operation (which is an example of “page erase operation” in the claims) will be described with reference to
FIG. 4A toFIGS. 4EA-4ED . -
FIG. 4A is a memory block circuit diagram for explaining a page erase operation. Although nine memory cells CL11 to CL33 in three rows and three columns are illustrated, the actual memory block is larger than this matrix. When memory cells are arranged in a matrix, one of the directions of the arrangement is called a row direction (or in rows) and the direction perpendicular to the one of the directions is called “column direction” (or in columns). To each of the memory cells, the source line SL, a corresponding one of the bit lines BL1 to BL3, a corresponding one of the plate lines PL1 to PL3, and a corresponding one of the word lines WL1 to WL3 are connected. For example, it is assumed that memory cells CL21 to CL23 to which the plate line PL2 and the word line WL2 are connected are selected in this block and a page erase operation is performed. - A mechanism of the page erase operation will be described with reference to
FIGS. 4BA -4BD andFIG. 4C . Here, thechannel region 7 between the N+ layers 3 a and 3b is electrically isolated from the substrate and functions as a floating body.FIG. 4BA is a timing operation waveform diagram of main nodes in the erase operation. InFIG. 4BA , T0 to T12 indicate times from the start to the end of the erase operation.FIG. 4BB illustrates a state at time T0 before the erase operation, in which the group ofpositive holes 9 generated by impact ionization in the previous cycle are stored in thechannel region 7. From time T1 to time T2, the bit lines BL1 to BL3 and the source line SL rise from Vss to VBLH and VSLH respectively and are in a high-voltage state. Here, Vss is, for example, equal to 0 V. With this operation, during the subsequent period from time T3 to time 14, the plate line PL2 selected in the page erase operation rises from a first voltage VPLL to a second voltage VPLH and is in a high-voltage state, the word line WL2 selected in the page erase operation rises from a third voltage Vss to a fourth voltage VWLH and is in a high-voltage state, and this prevents theinversion layer 12 a on the inner periphery of the firstgate conductor layer 5 a to which the plate line PL2 is connected and theinversion layer 12 b on the inner periphery of the secondgate conductor layer 5 b to which the word line WL2 is connected from being formed in thechannel region 7. Therefore, when the threshold voltage for the second N-channel MOS transistor region on the side of the word line WL2 and the threshold voltage for the first N-channel MOS transistor region on the side of the plate line PL2 are denoted by VtWL and VtPL respectively, it is desirable that the voltages VBLH and VSLH satisfy VBLH>VtWL and VSLH>VPLH+VtPL. For example, in a case where VtNL and VtPL are equal to 0.5 V, VWLH and VPLH need to be set to 3 V, and VBLH and VSLH need to be set to 3.5 V or higher. - The description of the mechanism of the page erase operation illustrated in
FIG. 4BA will be continued. As the plate line PL2 and the word line WL2 respectively rise to the second voltage VPLH and the fourth voltage VWLH and are in a high-voltage state during a first period from time T3 to time T4, the voltage of thechannel region 7 in a floating state is increased due to first capacitive coupling between the plate line PL2 and thechannel region 7 and the second capacitive coupling between the word line WL2 and thechannel region 7. The voltage of thechannel region 7 rises from VFB“1” in the “1” write state to a high voltage. This voltage rise is possible because the voltage of the bit lines BL1 to BL3 and that of the source line SL are high voltages of VBLH and VSLH respectively and the PN junction between the source N+ layer 3 a and thechannel region 7 and the PN junction between the drain N+ layer 3 b and thechannel region 7 are in a reverse bias state accordingly. - The description of the mechanism of the page erase operation illustrated in
FIG. 4BA will be continued. During the subsequent period from time T5 to time T6, the voltage of the bit lines BL1 to BL3 and that of the source line SL respectively drop from high voltages of VBLH and VSLH to Vss. As a result, the PN junction between the source N+ layer 3 a and thechannel region 7 and the PN junction between the drain N+ layer 3 b and thechannel region 7 are in a forward bias state as illustrated inFIG. 4BC , and a remaining group of positive holes among the group ofpositive holes 9 in thechannel region 7 are discharged to the source N+ layer 3 a and to the drain N+ layer 3 b. As a result, the voltage VFB of thechannel region 7 becomes equal to the built-in voltage Vb of the PN junction formed by the source N+ layer 3 a and the P layer, namely, thechannel region 7, and the PN junction formed by the drain N+ layer 3 b and the P layer, namely, thechannel region 7. - The description of the mechanism of the page erase operation illustrated in
FIG. 4BA will be continued. Subsequently, from time T7 to time T8, the voltage of the bit lines BL1 to BL3 and that of the source line SL rise from Vss to high voltages of VBLH and VSLB respectively. With this operation, as illustrated inFIG. 4BD , when the plate line PL2 drops from the second voltage VPLB to the first voltage VPLL and the word line WL2 drops from the fourth voltage VWLH to the third voltage Vss, from time T9 to time T10, the voltage VFB of thechannel region 7 efficiently changes from Vb to VFB“0” due to the first capacitive coupling between the plate line PL2 and thechannel region 7 and the second capacitive coupling between the word line WL2 and thechannel region 7 without theinversion layer 12 a on the side of the plate line PL2 or theinversion layer 12 b on the side of the word line WL2 being formed in thechannel region 7. The voltage difference ΔVFB of thechannel region 7 between the “1” write state and the “0” erase state is expressed by the following expressions. -
- Here, the sum of βWL and βPL is greater than or equal to 0.8, ΔVFB is large, and a sufficient margin is provided.
- As a result, as illustrated in
FIG. 4C , a large margin is provided between the “1” write state and the “0” erase state. Here, in the “0” erase state, the threshold voltage on the side of the plate line PL2 is high due to a substrate bias effect. Therefore, when the voltage applied to the plate line PL2 is set to, for example, the threshold voltage or lower, the first N-channel MOS transistor region on the side of the plate line PL2 becomes non-conducting and does not allow the memory cell current to flow therethrough. This state is illustrated in the right part ofFIG. 4C and indicated as “PL: non-conducting”. - The description of the mechanism of the page erase operation illustrated in
FIG. 4BA will be continued. During the subsequent period, namely, a fourth period from time T11 to time T12, the voltage of the bit lines BL1 to BL3 drops from VBLH to Vss and that of the source line SL drops from VSLH to Vss, and the erase operation ends. At this time, although the bit lines BL1 to BL3 and the source line SL slightly decrease the voltage of thechannel region 7 due to capacitive coupling, this decrease is equal to the increase in the voltage of thechannel region 7 by the bit lines BLi to BL3 and the source line SL due to capacitive coupling from time T7 to time T8, and therefore, the decrease and the increase in the voltage by the bit lines BL1 to BL3 and the source line SL are canceled out, and the voltage of thechannel region 7 is not affected consequently. The page erase operation in which the voltage VFB“0” in the “0” erase state of thechannel region 7 is assumed to be a second data retention voltage (which is an example of “second data retention voltage” in the claims) is performed to assign logical storage data “0”. When the voltage applied to the firstgate conductor layer 5 a connected to the plate line PL is set to a voltage higher than the threshold voltage at the time of logical storage data “1” and lower than the threshold voltage at the time of logical storage data “0” in data reading after the erase operation, a property that a current does not flow even when the voltage of the word line WL is increased can be attained as illustrated inFIG. 4C . - Now, a mechanism of the page erase operation will be described with reference to
FIGS. 4DA-4DD .FIGS. 4DA-4DD are different fromFIGS. 4BA-4BD in that the bit lines BL1 to BL3 are kept at Vss or put in a floating state and the word line WL2 is kept at Vss during the page erase operation. Accordingly, from time T1 to time T2, even when the source line SL rises from Vss to VSLH, the second N-channel MOS transistor region of the word line WL2 is non-conducting, and the memory cell current does not flow therethrough. Therefore, the group ofpositive holes 9 caused by an impact ionization phenomenon are not generated. The others are the same as inFIG. 4BA-4BD , and the source line SL changes between Vss and VSLH, and the plate line PL2 changes between VPLL and VPLH. As a result, as illustrated inFIG. 4DC , the group ofpositive holes 9 are discharged to the first impurity layer, namely, the N+ layer 3 a, of the source line SL. - Now, a mechanism of the page erase operation will be described with reference to
FIGS. 4EA-4ED .FIGS. 4EA-4ED are different fromFIGS. 4BA-4BD in that the source line SL is kept at Vss or put in a floating state and the plate line PL2 is kept at Vss during the page erase operation. Accordingly, from time T1 to time T2, even when the bit lines BL1 to BL3 rise from Vss to VBLH, the first N-channel MOS transistor region of the plate line PL2 is non-conducting, and the memory cell current does not flow therethrough. Therefore, the group ofpositive holes 9 caused by an impact ionization phenomenon are not generated. The others are the same as inFIGS. 4BA-4BD , and the bit lines BL1 to BL3 change between Vss and VBLH, and the word line WL2 changes between Vss and VWLH. As a result, as illustrated inFIG. 4EC , the group ofpositive holes 9 are discharged to the second impurity layer, namely, the N+ layer 3 b, of the bit lines BL1 to BL3. -
FIGS. 5A-5C are diagrams for explaining a read operation of the dynamic flash memory cell according to the first embodiment of the present invention. As illustrated inFIG. 5A , when thechannel region 7 is charged up to the built-in voltage Vb (about 0.7 V), the threshold voltage for the second N-channel MOS transistor region including the secondgate conductor layer 5 b to which the word line WL is connected decreases due to a substrate bias effect. This state is assigned to logical storage data “1”. As illustrated inFIG. 5B , a memory block selected before writing is in an erase state “0” in advance, and the voltage VFB of thechannel region 7 is equal to VFB“0”. With a write operation, a write state “1” is stored at random. As a result, logical storage data of logical “0” and that of logical “1” are created for the word line WL. As illustrated inFIG. 5C , the level difference between the two threshold voltages of the word line WL is used to perform reading by a sense amplifier. When the voltage applied to the firstgate conductor layer 5 a connected to the plate line PL is set to a voltage higher than the threshold voltage at the time of logical storage data “1” and lower than the threshold voltage at the time of logical storage data “0” in data reading, a property that a current does not flow even when the voltage of the word line WL is increased can be attained as illustrated inFIG. 5C . - A split-page write operation of the dynamic flash memory cell according to the first embodiment of the present invention will be described with reference to
FIG. 6A toFIG. 6E . -
FIG. 6A illustrates a part of a block (which is an example of “block” in the claims) in which a plurality of memory cells C00 to C53 are arranged in a matrix in four rows and six columns. In the actual memory block, the number of memory cells is larger than that of four rows and six columns. To each of the memory cells, a corresponding one of the word lines WL0 to WL3, a corresponding one of the plate lines PL0 to PL3, and a corresponding one of the bit lines BL0 to BL5 are connected. The bit lines BL0 to BL5 are respectively connected to sense amplifier circuits SA0 to SA5. - It is assumed in
FIG. 6A that the page erase operation described with reference toFIG. 4A toFIGS. 4EA -4ED is performed in advance and that the memory cells COO to C53 are in an “0” erase state. A case where, for example, all memory cells C01, C11, C21, C31, C41, and C51 included in a first page (which is an example of “first page” in the claims) perform a split-page write operation (which is an example of “split-page write operation” in the claims) will be specifically described. The split-page write operation is an operation in which a selected page for which the page erase operation is performed in advance is split into at least one or more sets of split pages (which is an example of “split pages” in the claims) and a page write operation is performed for all memory cells included in the selected page by performing the split-page write operation at least once. -
FIG. 6B is an operation waveform diagram of the split-page write operation, and the word line WL1 and the plate line PL1 connected to the memory cells C01, C11, C21, C31, C41, and C51 are selected. The period from time T1 to time T4 is assumed to be a first period (which is an example of “first period” in the claims) of the split-page write operation, and the period from time T5 to time T8 is assumed to be a second period (which is an example of “second period” in the claims) of the split-page write operation. - At time T1 in
FIG. 6B , the word line WL1 rises from a first voltage V1 to a second voltage V2. Here, the first voltage may be equal to, for example, the ground voltage Vss=0 V. When the word line WL1 rises from the first voltage V1 to the second voltage V2, the bit lines BL0 to BL5 are at a low voltage of a third voltage V3, and therefore, theinversion layer 12 b is formed in the secondchannel semiconductor layer 7 b. As a result, capacitive coupling between the word line WL1 and thechannel semiconductor layer 7 b is interrupted (shielded), and the group ofpositive holes 9 stored in the channel semiconductor layer of the memory cell to which “1” is written are prevented from being discharged through either thefirst impurity layer 3 a or thesecond impurity layer 3 b or both thefirst impurity layer 3 a and thesecond impurity layer 3 b. Here, the third voltage may be equal to, for example, the ground voltage Vss=0 V. - Subsequently, at time T2 in
FIG. 6B , the bit lines BL0 and BL2 rise from the third voltage V3 to a fourth voltage V4 in accordance with data, stored in advance in the sense amplifier circuits SA0 to SA2, for a first group of memory cells (which is an example of “first group of memory cells” in the claims) to be subjected to the split-page write operation. The word line WL1 rises to the second voltage V2 prior to the bit lines BL0 and BL2, and therefore, a “1” split-page write operation is performed in the memory cells C01 and C21. - Subsequently, at time T3 in
FIG. 6B , the bit lines BL0 and BL2 at the fourth voltage V4 drop to the third voltage V3. At this time point, the word line WL1 is at a high voltage of the second voltage V2. As a result, as the bit lines BL0 and BL2 drop, theinversion layer 12 b is formed in the secondchannel semiconductor layer 7 b. - At time T4, the word line WL1 drops from the second voltage V2 to the first voltage V1. The
inversion layer 12 b is present in the secondchannel semiconductor layer 7 b, capacitive coupling between the word line WL1 and thechannel semiconductor layer 7 b is interrupted (shielded), and the group ofpositive holes 9 stored in the channel semiconductor layers of the memory cells C01 and C21 to which “1” is written are prevented from being discharged through either thefirst impurity layer 3 a or thesecond impurity layer 3 b or both thefirst impurity layer 3 a and thesecond impurity layer 3 b. - Subsequently, at time T5 in
FIG. 6B , the word line WL1 rises again from the first voltage V1 to the second voltage V2. When the word line WL1 rises from the first voltage V1 to the second voltage V2, the bit lines BL0 to BL5 are at a low voltage of the third voltage V3, and therefore, theinversion layer 12 b is formed in the secondchannel semiconductor layer 7 b. As a result, capacitive coupling between the word line WL1 and thechannel semiconductor layer 7 b is interrupted (shielded), and the group ofpositive holes 9 stored in the channel semiconductor layer of the memory cell to which “1” is written are prevented from being discharged through either thefirst impurity layer 3 a or thesecond impurity layer 3 b or both thefirst impurity layer 3 a and thesecond impurity layer 3 b. - Subsequently, at time T6 in
FIG. 6B , the bit lines BL3 and BL4 rise from the third voltage V3 to the fourth voltage V4 in accordance with data, stored in advance in the sense amplifier circuits SA3 to SA5, for a second group of memory cells (which is an example of “second group of memory cells” in the claims) to be subjected to the split-page write operation. The word line WL1 rises to the second voltage V2 prior to the bit lines BL 3 and BL4, and therefore, a “1” split-page write operation is performed in the memory cells C31 and C41. - Subsequently, at time T7 in
FIG. 6B , the bit lines BL3 and BL4 at the fourth voltage V4 drop to the third voltage V3. At this time point, the word line WL1 is at a high voltage of the second voltage V2. As a result, as the bit lines BL3 and BL4 drop, theinversion layer 12 b is formed in the secondchannel semiconductor layer 7 b. - At time T8, the word line WL1 drops from the second voltage V2 to the first voltage V1. The
inversion layer 12 b is present in the secondchannel semiconductor layer 7 b, capacitive coupling between the word line WL1 and thechannel semiconductor layer 7 b is interrupted (shielded), and the group ofpositive holes 9 stored in the channel semiconductor layers of the memory cells C31 and C41 to which “1” is written are prevented from being discharged through either thefirst impurity layer 3 a or thesecond impurity layer 3 b or both thefirst impurity layer 3 a and thesecond impurity layer 3 b. - During the page write operation, the source line SL may be kept at a fifth voltage V5. Here, the fifth voltage V5 may be equal to, for example, the ground voltage Vss=0 V. The plate line PL1 that is the first driving control line may be kept at a sixth voltage V6.
- Although a case where the plate line PL1 is decoded and selected similarly to the word line WL1 has been described with reference to
FIG. 6A andFIG. 6B , the plate lines PL0 to PL3 may have a structure common to the block and need not be decoded. - Now, a split-page write operation for the first page and a second page (which is an example of “second page” in the claims) will be described with reference to
FIG. 6C . - A split-page write operation in which the memory cells C01, C11, C21, C31, C41, and C51 included in the first page and memory cells C02, C12, C22, C32, C42, and C52 included in the second page are alternately selected will be described with reference to the operation waveform diagram in
FIG. 6C . The period from time S1 to time S4 is assumed to be a third period (which is an example of “third period” in the claims) of the split-page write operation, and the period from time S5 to time S8 is assumed to be a fourth period (which is an example of “fourth period” in the claims) of the split-page write operation. - The split-page write operation in the first period from time T1 to time T4 in
FIG. 6C is the same as inFIG. 6B , and therefore, a description thereof will be omitted. In the split-page write operation in the third period, the word line WL2 rises from the first voltage V1 to the second voltage V2 at time S1. When the word line WL2 rises from the first voltage V1 to the second voltage V2, the bit lines BL0 to BL5 are at a low voltage of the third voltage V3, and therefore, theinversion layer 12 b is formed in the secondchannel semiconductor layer 7 b. As a result, capacitive coupling between the word line WL2 and thechannel semiconductor layer 7 b is interrupted (shielded), and the group ofpositive holes 9 stored in the channel semiconductor layer of the memory cell to which “1” is written are prevented from being discharged through either thefirst impurity layer 3 a or thesecond impurity layer 3 b or both thefirst impurity layer 3 a and thesecond impurity layer 3 b. - Subsequently, at time S2 in
FIG. 6C , the bit lines BL1 and BL2 rise from the third voltage V3 to the fourth voltage V4 in accordance with data for the split-page write operation for a third group of memory cells (which is an example of “third group of memory cells” in the claims) in the third period, the data being different from that in the first period and having being loaded to the sense amplifier circuits SA0 to SA2. The word line WL2 rises to the second voltage V2 prior to the bit lines BL1 and BL2, and therefore, a “1” split-page write operation is performed in the memory cells C12 and C22. - Subsequently, at time S3 in
FIG. 6C , the bit lines BL1 and BL2 at the fourth voltage V4 drop to the third voltage V3. At this time point, the word line WL2 is at a high voltage of the second voltage V2. As a result, as the bit lines BL1 and BL2 drop, theinversion layer 12 b, is formed in the secondchannel semiconductor layer 7 b. - At time S4, the word line WL2 drops from the second voltage V2 to the first voltage V1. The
inversion layer 12 b is present in the secondchannel semiconductor layer 7 b, capacitive coupling between the word line WL2 and thechannel semiconductor layer 7 b is interrupted (shielded), and the group ofpositive holes 9 stored in the channel semiconductor layers of the memory cells C12 and C22 to which “1” is written are prevented from being discharged through either thefirst impurity layer 3 a or thesecond impurity layer 3 b or both thefirst impurity layer 3 a and thesecond impurity layer 3 b. - Subsequently, at time T5 in
FIG. 6C , the word line WL1 rises again from the first voltage V1 to the second voltage V2. When the word line WL1 rises from the first voltage V1 to the second voltage V2, the bit lines BL0 to BL5 are at a low voltage of the third voltage V3, and therefore, theinversion layer 12 b, is formed in the secondchannel semiconductor layer 7 b. As a result, capacitive coupling between the word line WL1 and thechannel semiconductor layer 7 b is interrupted (shielded), and the group ofpositive holes 9 stored in the channel semiconductor layer of the memory cell to which “1” is written are prevented from being discharged through either thefirst impurity layer 3 a or thesecond impurity layer 3 b or both thefirst impurity layer 3 a and thesecond impurity layer 3 b. - Subsequently, at time T6 in
FIG. 6C , the bit lines BL3 and BL4 rise from the third voltage V3 to the fourth voltage V4 in accordance with data, for split-page writing in the second period, stored in advance in the sense amplifier circuits SA3 to SA5. The word line WL1 rises to the second voltage V2 prior to the bit lines BL3 and BL4, and therefore, a “1” split-page write operation is performed in the memory cells C31 and C41. - Subsequently, at time T7 in
FIG. 6C , the bit lines BL3 and BL4 at the fourth voltage V4 drop to the third voltage V3. At this time point, the word line WL1 is at a high voltage of the second voltage V2. As a result, as the bit lines BL3 and BL4 drop, theinversion layer 12 b, is formed in the secondchannel semiconductor layer 7 b. - At time T8, the word line WL1 drops from the second voltage V2 to the first voltage V1. The
inversion layer 12 b is present in the secondchannel semiconductor layer 7 b, capacitive coupling between the word line WL1 and thechannel semiconductor layer 7 b is interrupted (shielded), and the group ofpositive holes 9 stored in the channel semiconductor layers of the memory cells C31 and C41 to which “1” is written are prevented from being discharged through either thefirst impurity layer 3 a or thesecond impurity layer 3 b or both thefirst impurity layer 3 a and thesecond impurity layer 3 b. - Subsequently, at time S5 in
FIG. 6C , the word line WL2 rises again from the first voltage V1 to the second voltage V2. When the word line WL2 rises from the first voltage V1 to the second voltage V2, the bit lines BL0 to BL5 are at a low voltage of the third voltage V3, and therefore, theinversion layer 12 b, is formed in the secondchannel semiconductor layer 7 b. As a result, capacitive coupling between the word line WL2 and thechannel semiconductor layer 7 b is interrupted (shielded), and the group ofpositive holes 9 stored in the channel semiconductor layer of the memory cell to which “1” is written are prevented from being discharged through either thefirst impurity layer 3 a or thesecond impurity layer 3 b or both thefirst impurity layer 3 a and thesecond impurity layer 3 b. - Subsequently, at time S6 in
FIG. 6C , the bit lines BL3 and BL5 rise from the third voltage V3 to the fourth voltage V4 in accordance with data for the split-page write operation for a fourth group of memory cells (which is an example of “fourth group of memory cells” in the claims) in the fourth period, the data being different from that in the second period and having been loaded to the sense amplifier circuits SA3 to SA5. The word line WL2 rises to the second voltage V2 prior to the bit lines BL3 and BL5, and therefore, a “1” split-page write operation is performed in the memory cells C32 and C52. - Subsequently, at time S7 in
FIG. 6C , the bit lines BL3 and BL5 at the fourth voltage V4 drop to the third voltage V3. At this time point, the word line WL2 is at a high voltage of the second voltage V2. As a result, as the bit lines BL3 and BL5 drop, theinversion layer 12 b, is formed in the secondchannel semiconductor layer 7 b. - At time S8, the word line WL2 drops from the second voltage V2 to the first voltage V1. The
inversion layer 12 b is present in the secondchannel semiconductor layer 7 b, capacitive coupling between the word line WL2 and thechannel semiconductor layer 7 b is interrupted (shielded), and the group ofpositive holes 9 stored in the channel semiconductor layers of the memory cells C32 and C52 to which “1” is written are prevented from being discharged through either thefirst impurity layer 3 a or thesecond impurity layer 3 b or both thefirst impurity layer 3 a and thesecond impurity layer 3 b. -
FIG. 6D illustrates an example in which in a state in which the word line WL1 and the plate line PL1 of the first page are selected, in the first period, a first group of bit lines BL0 to BL2 (which is an example of “first group of bit lines” in the claims) is selected and the split-page write operation of the first group of memory cells to which the first group of bit lines is connected is performed, and in the second period, a second group of bit lines BL3 to BL5 (which is an example of “second group of bit lines” in the claims) is selected and the split-page write operation of the second group of memory cells to which the second group of bit lines is connected is performed. -
FIG. 6E illustrates an example in which in split-page writing of the first page, the plate line PL1 is clock-operated similarly to the word line WL1. In the first period and in the second period, the plate line PL1 rises from a low voltage of the sixth voltage V6 to a high voltage of a seventh voltage V7 that is higher than the sixth voltage V6. - Regardless of whether the horizontal cross-sectional shape of the
Si pillar 2 illustrated inFIG. 1 is a round shape, an elliptic shape, or a rectangular shape, the operations of the dynamic flash memory described in this embodiment can be performed. Further, a dynamic flash memory cell having a round shape, a dynamic flash memory cell having an elliptic shape, and a dynamic flash memory cell having a rectangular shape may coexist on the same chip. - With reference to
FIG. 1 , the dynamic flash memory element including an example SGT in which the firstgate insulating layer 4 a and the secondgate insulating layer 4 b that surround the entire side surface of theSi pillar 2 standing on the substrate in the vertical direction are provided and which includes the firstgate conductor layer 5 a and the secondgate conductor layer 5 b that entirely surround the firstgate insulating layer 4 a and the secondgate insulating layer 4 b has been described. As illustrated in the description of this embodiment, the dynamic flash memory element needs to have a structure that satisfies the condition that the group ofpositive holes 9 generated by an impact ionization phenomenon are retained in thechannel region 7. For this, thechannel region 7 needs to have a floating body structure isolated from the substrate. Accordingly, even when the semiconductor base material of the channel region is formed horizontally along the substrate by using, for example, GAA (Gate All Around, see, for example, NPL 11) technology, which is one type of SGT, or nanosheet technology (see, for example, NPL 12), the above-described operations of the dynamic flash memory can be performed. Alternatively, the dynamic flash memory element may have a device structure using SOI (Silicon On Insulator) (see, for example,NPL 7 to NPL 10). In this device structure, the bottom portion of the channel region is in contact with an insulating layer of the SOI substrate, and the other portion of the channel region is surrounded by a gate insulating layer and an element isolation insulating layer. Also with such a structure, the channel region has a floating body structure. Accordingly, the dynamic flash memory element provided in this embodiment needs to satisfy the condition that the channel region has a floating body structure. Even with a structure in which a Fin transistor (see, for example, NPL 13) is formed on an SOI substrate, when the channel region has a floating body structure, the dynamic flash operations can be performed. - To write “1”, electron-positive hole pairs may be generated by using a gate-induced drain leakage (GIDL) current (see for example, NPL 14), and the
channel region 7 may be filled with the generated group of positive holes. - Expressions (1) to (12) provided in the specification and in the drawings are expressions used to qualitatively explain the phenomena, and are not intended to limit the phenomena.
- Although the reset voltages of the word line WL, the bit line BL, and the source line SL are specified as Vss in the explanations of
FIGS. 3AA-3AC andFIG. 3B , the reset voltages may be set to different voltages. - Although
FIGS. 4BA-4BD ,FIGS. 4DA-4DD , andFIGS. 4EA-4ED illustrate example conditions of the page erase operation, the voltages applied to the source line SL, the plate line PL, the bit line BL, and the word line WL may be changed as long as a state in which the group ofpositive holes 9 in thechannel region 7 are discharged through either the N+ layer 3 a or the N+ layer 3 b or both the N+ layer 3 a and the N+ layer 3 b can be attained. Further, in the page erase operation, a voltage may be applied to the source line SL of a selected page, and the bit line BL may be put in a floating state. In the page erase operation, a voltage may be applied to the bit line BL of a selected page, and the source line SL may be put in a floating state. - In
FIG. 1 , in the vertical direction, in a part of thechannel region 7 surrounded by the insulatinglayer 6 that is the first insulating layer, the potential distribution of the first channel region 7 a and that of thesecond channel region 7 b are connected and formed. Accordingly, the first channel region 7 a and thesecond channel region 7 b that constitute thechannel region 7 are connected in the vertical direction in the region surrounded by the insulatinglayer 6 that is the first insulating layer. - Note that in
FIG. 1 , it is desirable to make the length of the firstgate conductor layer 5 a, in the vertical direction, to which the plate line PL is connected further longer than the length of the secondgate conductor layer 5 b, in the vertical direction, to which the word line WL is connected to attain CPL>CWL. However, when only the plate line PL is added, the coupling ratio (CWL/(CPL+CWL+CBL+CSL)), of capacitive coupling, of the word line WL to thechannel region 7 decreases. As a result, the potential change ΔVFB of the floating body, namely, thechannel region 7, decreases. - As the voltage VPLL of the plate line PL, a fixed voltage of, for example, 2 V may be applied in operation modes other than a mode in which selective erasing is performed in a block erase operation.
- Note that in the specification and the claims, the meaning of “cover” in a case of “for example, a gate insulating layer or a gate conductor layer covers, for example, a channel” also includes a case of surrounding entirely as in an SGT or GAA, a case of surrounding except a portion as in a Fin transistor, and a case of overlapping a flat object as in a planar transistor.
- Although the first
gate conductor layer 5 a entirely surrounds the firstgate insulating layer 4 a inFIG. 1 , a structure may be employed in which the firstgate conductor layer 5 a partially surrounds the firstgate insulating layer 4 a in plan view. In this case, the outer side of the first gate insulating layer not covered by the firstgate conductor layer 5 a may be covered by an insulating layer or a third gate conductor layer electrically isolated from the first gate conductor layer. In a case where the third gate conductor layer is provided, a constant voltage or a pulse voltage can be applied to the third gate conductor layer to perform the operations of the dynamic flash memory. With the structure as described above in which the firstgate conductor layer 5 a partially surrounds the firstgate insulating layer 4 a in plan view, a group of a larger number of positive holes can be stored in the first channel region 7 a. - Although the channel region 7 a is entirely surrounded by the first
gate conductor layer 5 a inFIG. 1 , the firstgate conductor layer 5 a may be divided into two or more gate conductor layers, and each of the gate conductor layers may function as a conductive electrode of the plate line and surround the channel region 7 a. The two or more gate conductor layers obtained as a result of division may be operated synchronously or asynchronously at the same driving voltage or different driving voltages. Similarly, the secondgate conductor layer 5 b may be divided into two or more gate conductor layers, and the gate conductor layers may each function as a conductive electrode of the word line and may be operated synchronously or asynchronously at the same driving voltage or different driving voltages. Also in this case, the operations of the dynamic flash memory can be performed. In a case where the firstgate conductor layer 5 a is divided into two or more gate conductor layers, at least one of the first gate conductor layers obtained as a result of division assumes the roles of the firstgate conductor layer 5 a described above. In a case where the secondgate conductor layer 5 b is divided into two or more gate conductor layers, at least one of the second gate conductor layers obtained as a result of division assumes the roles of the secondgate conductor layer 5 b described above. - In
FIG. 1 , the firstgate conductor layer 5 a may be connected to the word line WL, and the secondgate conductor layer 5 b may be connected to the plate line PL. Also in this case, the operations of the dynamic flash memory described above can be performed. - The above-described conditions of voltages applied to the bit line BL, the source line SL, the word line WL, and the plate line PL and the voltage of the floating body are examples for performing basic operations including the erase operation, the write operation, and the read operation, and other voltage conditions may be employed as long as basic operations of the present invention can be performed.
- This embodiment has the following features.
- The dynamic flash memory cell of this embodiment is constituted by the N+ layers 3 a and 3b that function as the source and the drain, the
channel region 7, the firstgate insulating layer 4 a, the secondgate insulating layer 4 b, the firstgate conductor layer 5 a, and the secondgate conductor layer 5 b, which are formed in a pillar form as a whole. The N+ layer 3 a that functions as the source is connected to the source line SL, the N+ layer 3 b that functions as the drain is connected to the bit line BL, the firstgate conductor layer 5 a is connected to the plate line PL, and the secondgate conductor layer 5 b is connected to the word line WL. A structure is employed in which the gate capacitance of the firstgate conductor layer 5 a to which the plate line PL is connected is larger than the gate capacitance of the secondgate conductor layer 5 b to which the word line WL is connected, which is a feature. In the dynamic flash memory cell, the first gate conductor layer and the second gate conductor layer are stacked in the vertical direction. Accordingly, even when the structure is employed in which the gate capacitance of the firstgate conductor layer 5 a to which the plate line PL is connected is larger than the gate capacitance of the secondgate conductor layer 5 b to which the word line WL is connected, the memory cell area does not increase in plan view. Accordingly, a high-performance and highly integrated dynamic flash memory cell can be implemented. - With the split-page write operation of the dynamic flash memory cell according to the first embodiment of the present invention, the user can select a data size for the split-page write operation as desired within the range of the number of memory cells (page size) connected to the page. The group of memory cells for the split-page write operation can be easily split, a selected page is changed among a plurality of different pages, and the split-page write operation can be performed for different groups of memory cells. The DRAM that is a nonvolatile memory in the related art is a read-destructive-type memory, and therefore, writing needs to be performed simultaneously for all memory cells in a selected page also in a page write operation. With the split-page write operation of the dynamic flash memory cell of the present invention, the system flexibility is significantly increased for users who handle various data capacities. As a result, a high-speed system can be attained.
- In the page write operation and the split-page write operation of the dynamic flash memory cell according to the first embodiment of the present invention, the word line WL rises prior to the bit line BL. As a result, the
inversion layer 12 b, is formed in thechannel semiconductor layer 7 of the memory cell to which “1” is written, to interrupt (shield) capacitive coupling between the word line WL and thechannel semiconductor layer 7. Therefore, some of the group ofpositive holes 9 stored in thechannel semiconductor layer 7 are not discharged, and a highly reliable memory device can be provided. - In terms of the roles of the first
gate conductor layer 5 a to which the plate line PL is connected in the dynamic flash memory cell according to the first embodiment of the present invention, in the write operation and in the read operation performed by the dynamic flash memory cell, the voltage of the word line WL changes. At this time, the plate line PL assumes the role of decreasing the capacitive coupling ratio between the word line WL and thechannel region 7. As a result, an effect on changes in the voltage of thechannel region 7 when the voltage of the word line WL changes can be substantially suppressed. Accordingly, the difference between the threshold voltages for the transistor regions of the word line WL indicating logical “0” and logical “1” can be increased. This leads to an increased operation margin of the dynamic flash memory cell. - In
FIG. 6A , the plate line common to the memory cells C00 to C53 may be used. As a result, the processes and circuits can be more simplified, and a higher speed can be attained. - Although the Si pillar is formed in the present invention, the Si pillar may be a semiconductor pillar made of a semiconductor material other than Si. This is similarly applicable to other embodiments according to the present invention.
- In vertical NAND-type flash memory circuits, memory cells that are stacked in a plurality of stages in the vertical direction and each of which is constituted by a semiconductor pillar, which functions as the channel, and a tunnel oxide layer, a charge storage layer, an interlayer insulating layer, and a control conductor layer that surround the semiconductor pillar are formed. At the semiconductor pillars on both ends of these memory cells, a source line impurity layer corresponding to the source and a bit line impurity layer corresponding to the drain are disposed respectively. In addition, for one memory cell, when one of the memory cells on both sides of the one memory cell functions as the source, the other functions as the drain. Accordingly, the vertical NAND-type flash memory circuit is one type of SGT circuit. Therefore, the present invention is also applicable to a circuit in which a NAND-type flash memory circuit coexists.
- To write “1”, electron-positive hole pairs may be generated by an impact ionization phenomenon using a gate-induced drain leakage (GIDL) current described in
NPL 10 and NPL 14, and the floating body FB may be filled with the generated group of positive holes. This is similarly applicable to other embodiments according to the present invention. - Even with a structure in which the conductivity type that is the polarity of each of the N+ layers 3 a and 3b and the P-
layer Si pillar 2 inFIG. 1 is reversed, the operations of the dynamic flash memory can be performed. In this case, in theSi pillar 2 that is of N-type, the majority carriers are electrons. Therefore, a group of electrons generated by impact ionization are stored in thechannel region 7, and a “1” state is set. - Various embodiments and modifications can be made to the present invention without departing from the spirit and scope of the present invention in a broad sense. The above-described embodiments are intended to explain examples of the present invention and are not intended to limit the scope of the present invention. Any of the above-described embodiments and modifications can be combined. Further, the above-described embodiments from which some of the constituent requirements are removed as needed are also within the scope of the technical spirit of the present invention.
- With the memory device including a semiconductor element according to the present invention, a high-density and high-performance dynamic flash memory that is a memory device including an SGT can be obtained.
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US20150117113A1 (en) * | 2013-10-24 | 2015-04-30 | Western Digital Technologies, Inc. | Programming scheme for improved voltage distribution in solid-state memory |
US20170330623A1 (en) * | 2016-05-12 | 2017-11-16 | Korea University Research And Business Foundation | Dual gate semiconductor memory device with vertical semiconductor column |
US20220139437A1 (en) * | 2020-10-30 | 2022-05-05 | Ferroelectric Memory Gmbh | Memory cell arrangement and method thereof |
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US20150117113A1 (en) * | 2013-10-24 | 2015-04-30 | Western Digital Technologies, Inc. | Programming scheme for improved voltage distribution in solid-state memory |
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